Use the Docker build cache in the ESS test suite

Import the Docker build cache that the Build workflow pushes to ghcr.io on
main, which caches the toolchain, frontend and policy layers

.github/workflows/ci.yaml

@@ -380,10 +380,19 @@ jobs:
         with:
           ess-helm-ref: ${{ steps.check-branch.outputs.ref }}
 
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@d7f5e7f509e45cec5c76c4d5afdd7de93d0b3df5 # v4.1.0
+        with:
+          buildkitd-config-inline: |
+            [registry."docker.io"]
+              mirrors = ["mirror.gcr.io"]
+
       - name: Build MAS docker image to local registry
-        run: |
-          docker build -t localhost:5000/matrix-authentication-service:ci -f ./Dockerfile .
-          docker push localhost:5000/matrix-authentication-service:ci
+        uses: docker/build-push-action@53b7df96c91f9c12dcc8a07bcb9ccacbed38856a # v7.3.0
+        with:
+          cache-from: type=registry,ref=ghcr.io/element-hq/matrix-authentication-service/buildcache:buildcache-amd64
+          push: true
+          tags: localhost:5000/matrix-authentication-service:ci
 
       - name: Build Additional values file
         run: >