Add docker-build-cached recipe for CI with BuildKit layer caching

CI uses docker-build-cached (BuildKit with GHA cache) for faster
builds. Local development uses docker-build (plain Docker). Both
are in the justfile — CI still uses just recipes exclusively.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bricef

.github/workflows/ci.yml

@@ -33,6 +33,9 @@ jobs:
 
       - uses: extractions/setup-just@v2
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
       - name: Log in to Docker Hub
         uses: docker/login-action@v3
         with:
@@ -43,6 +46,6 @@ jobs:
         env:
           DOCKER_REPO: ${{ secrets.DOCKERHUB_USERNAME }}/taskflow
         run: |
-          just docker-build
+          just docker-build-cached
           just docker-push latest
           just docker-push ${{ github.sha }}

justfile

@@ -54,6 +54,12 @@ run:
 docker-build:
     docker build -t taskflow .
 
+# Build Docker image with BuildKit cache (used in CI)
+docker-build-cached:
+    docker buildx build --load \
+      --cache-from type=gha --cache-to type=gha,mode=max \
+      -t taskflow .
+
 # Push Docker image to registry
 docker-push tag="latest":
     docker tag taskflow {{DOCKER_REPO}}:{{tag}}