GitHub Actions Docker build ignoring cache and rebuilding from scratch every run

Every GitHub Actions run was rebuilding the Docker image from layer 1, even though nothing in the Dockerfile had changed. The cache-from flag was set but having zero effect — build times stayed at 4+ minutes every run.

Root cause: I was using actions/cache to cache Docker layers manually, but this approach has a subtle bug: the cache key was tied to the branch name, so PRs from forks and new branches always missed. The real fix is using docker/build-push-action with GitHub Actions native cache (type=gha), which handles key management automatically and works across branches.

Fix:

Replace the manual cache setup with this workflow step:

yaml

- name: Set up Docker Buildx
  uses: docker/setup-buildx-action@v3
 
- name: Build and push
  uses: docker/build-push-action@v5
  with:
    context: .
    push: true
    tags: myrepo/myapp:latest
    cache-from: type=gha
    cache-to: type=gha,mode=max

The mode=max on cache-to caches all intermediate layers, not just the final image. Without it you still get cache misses on early layers.

Remove any manual actions/cache steps for Docker — they conflict with type=gha.

After this change, unchanged builds dropped from 4 minutes to under 30 seconds.

Lesson: Use docker/build-push-action with cache-from: type=gha and cache-to: type=gha,mode=max — manual actions/cache for Docker layers is fragile and almost never works correctly.