Try local Docker image caching in GHA

.github/workflows/test.yml

@@ -58,12 +58,32 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v2
         with:
-          driver: docker
           install: true
 
+      # Enable Docker layer caching without having to push to a registry.
+      # https://github.com/docker/build-push-action/blob/master/docs/advanced/cache.md#local-cache
+      # This may eventually be migrated to the GitHub Actions cache backend,
+      # which is still considered experimental.
+      # https://github.com/moby/buildkit#github-actions-cache-experimental
+      - name: Cache Docker layers
+        uses: actions/cache@v2
+        with:
+          path: /tmp/.buildx-cache
+          key: ${{ matrix.platform }}-buildx-${{ github.sha }}
+          restore-keys: ${{ matrix.platform }}-buildx-
+
+      # Use docker buildx instead of docker-compose here because cache exporting
+      # does not seem to work as of docker-compose v2.6.0 and buildx v0.8.2, even
+      # though it works with buildx individually.
       - name: Build R
         run: |
           for version in ${{ env.R_VERSIONS }}; do
+            docker buildx build -t r-builds:${{ matrix.platform }} \
+              --file builder/Dockerfile.${{ matrix.platform }} \
+              --cache-from "type=local,src=/tmp/.buildx-cache" \
+              --cache-to "type=local,dest=/tmp/.buildx-cache-new,mode=max" \
+              --load \
+              builder
             R_VERSION=$version make build-r-${{ matrix.platform }}
           done
 
@@ -72,3 +92,11 @@ jobs:
           for version in ${{ env.R_VERSIONS }}; do
             R_VERSION=$version make test-r-${{ matrix.platform }}
           done
+
+      # Temporary workaround for unbounded GHA cache growth with the local cache mode.
+      # https://github.com/docker/build-push-action/issues/252
+      # https://github.com/moby/buildkit/issues/1896
+      - name: Move cache
+        run: |
+          rm -rf /tmp/.buildx-cache
+          mv /tmp/.buildx-cache-new /tmp/.buildx-cache