Docker build cache not being used in CI — every build starting from scratch | Today I Fixed

The problem: Docker builds in GitHub Actions were taking 8 minutes every run. The cache layers were being pulled but not used — every step showed "CACHED" in local builds but not in CI.

The fix:

yaml

# GitHub Actions workflow — add cache-from and cache-to:
- name: Build and push Docker image
  uses: docker/build-push-action@v5
  with:
    context: .
    push: true
    tags: my-app:latest
    cache-from: type=gha          # Pull cache from GitHub Actions cache
    cache-to: type=gha,mode=max   # Push all layers to cache (not just final)

dockerfile

# Also make sure Dockerfile is ordered for cache efficiency:
# WRONG order — code changes invalidate all subsequent layers:
COPY . .
RUN npm install
 
# RIGHT order — dependencies cached separately from code:
COPY package*.json ./
RUN npm install        # ← cached unless package.json changes
COPY . .               # ← only this layer rebuilds on code change
RUN npm run build

Why it happens: GitHub Actions runners are ephemeral — each run starts on a fresh machine with no local Docker cache. Without cache-from: type=gha, Docker has nothing to compare against and rebuilds everything. The mode=max flag exports intermediate layers too (not just the final image), making subsequent builds much faster.