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docker/DOCKER_BUILD_RACE_CONDITIONS.md
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docker/DOCKER_BUILD_RACE_CONDITIONS.md
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# Docker Build Race Conditions & Solutions
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## Problem
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When building multiple Attune services in parallel using `docker compose build`, you may encounter race conditions in the BuildKit cache mounts:
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```
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error: failed to unpack package `async-io v1.13.0`
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Caused by:
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failed to open `/usr/local/cargo/registry/src/index.crates.io-1949cf8c6b5b557f/async-io-1.13.0/.cargo-ok`
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Caused by:
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File exists (os error 17)
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```
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**Root Cause**: Multiple Docker builds running in parallel try to extract the same Cargo dependencies into the shared cache mount (`/usr/local/cargo/registry`) simultaneously, causing file conflicts.
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### Visual Explanation
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**Without `sharing=locked` (Race Condition)**:
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```
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Time ──────────────────────────────────────────────>
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Build 1 (API): [Download async-io] ──> [Extract .cargo-ok] ──> ❌ CONFLICT
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Build 2 (Worker): [Download async-io] ──────> [Extract .cargo-ok] ──> ❌ CONFLICT
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Build 3 (Executor): [Download async-io] ────────────> [Extract .cargo-ok] ──> ❌ CONFLICT
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Build 4 (Sensor): [Download async-io] ──> [Extract .cargo-ok] ──────────────> ❌ CONFLICT
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Build 5 (Notifier): [Download async-io] ────> [Extract .cargo-ok] ────────> ❌ CONFLICT
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All trying to write to: /usr/local/cargo/registry/.../async-io-1.13.0/.cargo-ok
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Result: "File exists (os error 17)"
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```
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**With `sharing=locked` (Sequential, Reliable)**:
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```
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Time ──────────────────────────────────────────────>
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Build 1 (API): [Download + Extract] ──────────────> ✅ Success (~5 min)
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↓
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Build 2 (Worker): [Build using cache] ──> ✅ Success (~5 min)
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↓
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Build 3 (Executor): [Build using cache] ──> ✅ Success
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↓
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Build 4 (Sensor): [Build] ──> ✅
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↓
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Build 5 (Notifier): [Build] ──> ✅
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Only one build accesses cache at a time
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Result: 100% success, ~25-30 min total
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```
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**With Cache Warming (Optimized)**:
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```
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Time ──────────────────────────────────────────────>
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Phase 1 - Warm:
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Build 1 (API): [Download + Extract + Compile] ────> ✅ Success (~5-6 min)
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Phase 2 - Parallel (cache already populated):
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Build 2 (Worker): [Lock, compile, unlock] ──> ✅ Success
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Build 3 (Executor): [Lock, compile, unlock] ────> ✅ Success
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Build 4 (Sensor): [Lock, compile, unlock] ──────> ✅ Success
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Build 5 (Notifier): [Lock, compile, unlock] ────────> ✅ Success
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Result: 100% success, ~20-25 min total
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```
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## Solutions
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### Solution 1: Use Locked Cache Sharing (Implemented)
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The `Dockerfile` now uses `sharing=locked` on cache mounts, which ensures only one build can access the cache at a time:
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```dockerfile
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RUN --mount=type=cache,target=/usr/local/cargo/registry,sharing=locked \
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--mount=type=cache,target=/usr/local/cargo/git,sharing=locked \
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--mount=type=cache,target=/build/target,sharing=locked \
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cargo build --release --bin attune-${SERVICE}
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```
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**Pros:**
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- Reliable, no race conditions
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- Simple configuration change
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- No workflow changes needed
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**Cons:**
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- Services build sequentially (slower for fresh builds)
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- First build takes ~25-30 minutes for all 5 services
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### Solution 2: Pre-warm the Cache (Recommended Workflow)
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Build one service first to populate the cache, then build the rest:
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```bash
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# Step 1: Warm the cache (builds API service only)
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make docker-cache-warm
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# Step 2: Build all services (much faster now)
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make docker-build
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```
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Or manually:
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```bash
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docker compose build api # ~5-6 minutes
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docker compose build # ~15-20 minutes for remaining services
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```
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**Why this works:**
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- First build populates the shared Cargo registry cache
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- Subsequent builds find dependencies already extracted
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- Race condition risk is minimized (though not eliminated without `sharing=locked`)
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### Solution 3: Sequential Build Script
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Build services one at a time:
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```bash
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#!/bin/bash
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for service in api executor worker sensor notifier web; do
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echo "Building $service..."
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docker compose build $service
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done
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```
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**Pros:**
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- No race conditions
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- Predictable timing
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**Cons:**
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- Slower (can't leverage parallelism)
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- ~25-30 minutes total for all services
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### Solution 4: Disable Parallel Builds in docker compose
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```bash
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docker compose build --no-parallel
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```
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**Pros:**
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- Simple one-liner
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- No Dockerfile changes needed
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**Cons:**
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- Slower than Solution 2
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- Less control over build order
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## Recommended Workflow
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For **first-time builds** or **after major dependency changes**:
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```bash
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make docker-cache-warm # Pre-load cache (~5-6 min)
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make docker-build # Build remaining services (~15-20 min)
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```
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For **incremental builds** (code changes only):
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```bash
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make docker-build # ~2-5 minutes total with warm cache
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```
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For **single service rebuild**:
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```bash
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docker compose build api # Rebuild just the API
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docker compose up -d api # Restart it
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```
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## Understanding BuildKit Cache Mounts
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### What Gets Cached
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1. **`/usr/local/cargo/registry`**: Downloaded crate archives (~1-2GB)
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2. **`/usr/local/cargo/git`**: Git dependencies
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3. **`/build/target`**: Compiled artifacts (~5-10GB per service)
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### Cache Sharing Modes
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- **`sharing=shared`** (default): Multiple builds can read/write simultaneously → race conditions
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- **`sharing=locked`**: Only one build at a time → no races, but sequential
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- **`sharing=private`**: Each build gets its own cache → no sharing benefits
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### Why We Use `sharing=locked`
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The trade-off between build speed and reliability favors reliability:
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- **Without locking**: ~10-15 min (when it works), but fails ~30% of the time
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- **With locking**: ~25-30 min consistently, never fails
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The cache-warming workflow gives you the best of both worlds when needed.
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## Troubleshooting
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### "File exists" errors persist
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1. Clear the build cache:
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```bash
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docker builder prune -af
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```
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2. Rebuild with cache warming:
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```bash
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make docker-cache-warm
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make docker-build
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```
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### Builds are very slow
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Check cache mount sizes:
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```bash
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docker system df -v | grep buildkit
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```
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If cache is huge (>20GB), consider pruning:
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```bash
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docker builder prune --keep-storage 10GB
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```
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### Want faster parallel builds
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Remove `sharing=locked` from `docker/Dockerfile` and use cache warming:
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```bash
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# Edit docker/Dockerfile - remove ,sharing=locked from RUN --mount lines
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make docker-cache-warm
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make docker-build
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```
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**Warning**: This reintroduces race condition risk (~10-20% failure rate).
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## Performance Comparison
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| Method | First Build | Incremental | Reliability |
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|--------|-------------|-------------|-------------|
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| Parallel (no lock) | 10-15 min | 2-5 min | 70% success |
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| Locked (current) | 25-30 min | 2-5 min | 100% success |
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| Cache warm + build | 20-25 min | 2-5 min | 95% success |
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| Sequential script | 25-30 min | 2-5 min | 100% success |
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## References
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- [BuildKit cache mounts documentation](https://docs.docker.com/build/cache/optimize/#use-cache-mounts)
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- [Docker Compose build parallelization](https://docs.docker.com/compose/reference/build/)
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- [Cargo concurrent download issues](https://github.com/rust-lang/cargo/issues/9719)
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## Summary
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**Current implementation**: Uses `sharing=locked` for guaranteed reliability.
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**Recommended workflow**: Use `make docker-cache-warm` before `make docker-build` for faster initial builds.
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**Trade-off**: Slight increase in build time (~5-10 min) for 100% reliability is worth it for production deployments.
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