divine-mobile: Skill for Claude Code

Docker Buildx Stale Rust Binary Cache

Problem

Docker buildx may serve stale compiled Rust binaries from layer cache even when source files have changed. The image gets a new tag and pushes successfully, but contains the old compiled binary. This is especially common with cross-platform builds (--platform linux/amd64 on ARM Macs).

Context / Trigger Conditions

• Dockerfile uses multi-stage pattern: copy Cargo.toml → build deps → copy source → build
• Using docker buildx build --push with --platform linux/amd64
• Code changes verified locally (tests pass) but production shows old behavior
• Image digest from cached build differs from --no-cache build
• Adding a simple response header in code and it doesn't appear in production

Solution

Always use --no-cache when deploying code changes:

# WRONG — may use cached compilation layer with old code
docker buildx build --platform linux/amd64 --target api \
  -t registry/image:tag --push .

# CORRECT — forces full recompilation
docker buildx build --platform linux/amd64 --target api \
  --no-cache -t registry/image:tag --push .

Verification

Compare image digests between cached and no-cache builds:

# Build with cache
docker buildx build --platform linux/amd64 --target api \
  -t registry/image:cached --push . 2>&1 | grep "pushing manifest"
# Note the sha256 digest

# Build without cache
docker buildx build --platform linux/amd64 --target api \
  --no-cache -t registry/image:nocache --push . 2>&1 | grep "pushing manifest"
# Note the sha256 digest

# If digests differ, the cached build had stale code

Example

Typical Dockerfile pattern vulnerable to this:

# Layer 1: Copy manifests (cached if Cargo.toml unchanged)
COPY Cargo.toml Cargo.lock ./
COPY crates/*/Cargo.toml crates/

# Layer 2: Build dependencies (cached — this is the good cache)
RUN cargo build --release && rm -rf src crates

# Layer 3: Copy actual source (SHOULD invalidate on source change)
COPY crates crates
COPY bin bin

# Layer 4: Rebuild with actual source
RUN touch crates/*/src/lib.rs && cargo build --release