Skip image build/push if it exists

To avoid building images on every merge to main, check if image exists
first and only build/push if absent.

.github/workflows/publish-containers.yml

@@ -35,40 +35,52 @@ jobs:
         platform:
           - linux/amd64,linux/arm64
     steps:
+      - name: Log in to the Container registry
+        uses: docker/login-action@v2
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Extract metadata for Lotus ${{ matrix.version }} ${{ matrix.net.name }}
+        id: meta
+        uses: docker/metadata-action@v4
+        with:
+          images: ghcr.io/${{ github.repository }}
+          tags: |
+            type=raw,value=lotus-${{ matrix.version }}-${{ matrix.net.name }}
+      - name: Check if image exists
+        id: checker
+        run: |
+          if ! docker manifest inspect ${{ steps.meta.outputs.tags }}
+          then
+            echo "IMAGE_EXISTS=false" >> $GITHUB_ENV
+          fi
       - name: Free Disk Space (Ubuntu)
         uses: jlumbroso/free-disk-space@main
+        if: env.IMAGE_EXISTS == 'false'
         with:
          tool-cache: false
          android: true
          dotnet: true
          haskell: true
       - name: Checkout
         uses: actions/checkout@v3
+        if: env.IMAGE_EXISTS == 'false'
         with:
           repository: filecoin-project/lotus
           ref: ${{ matrix.version }}
           path: lotus
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v2
+        if: env.IMAGE_EXISTS == 'false'
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v2
+        if: env.IMAGE_EXISTS == 'false'
         with:
           platforms: ${{ matrix.platform }}
-      - name: Log in to the Container registry
-        uses: docker/login-action@v2
-        with:
-          registry: ghcr.io
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
-      - name: Extract metadata for Lotus ${{ matrix.version }} ${{ matrix.net.name }}
-        id: meta
-        uses: docker/metadata-action@v4
-        with:
-          images: ghcr.io/${{ github.repository }}
-          tags: |
-            type=raw,value=lotus-${{ matrix.version }}-${{ matrix.net.name }}
       - name: Build Lotus ${{ matrix.version }} ${{ matrix.net.name }}
         uses: docker/build-push-action@v4
+        if: env.IMAGE_EXISTS == 'false'
         env:
           REGISTRY_CACHE_REF: ghcr.io/${{ github.repository }}:cache
         with: