feat: add guards for absent registry credentials

The original design assumed that every user would configure login credentials with appropriate write permissions for the Docker Hub repository.  Consequently, forks or pull requests fail on the first step.

This change allows the build to complete without login credentials.  It skips pushing the cache or pushing the candidate image.

The release (docker tag) step still requires login credentials and fails if they are absent.  This guarantees that git tagging is only possible once all images have been pushed out to all container registries.

PR: #24

.github/workflows/containers.yml

@@ -22,13 +22,25 @@ jobs:
           - dev
           - build
     steps:
+      - name: Check for GitHub Container Registry credentials (secrets)
+        id: ghcr-credentials
+        run: if [ ${{ secrets.GHCR_USERNAME == null || secrets.GHCR_TOKEN == null }} = true ]; then exit 1; fi
+        continue-on-error: true
       - name: Login to GitHub Container Registry
+        id: ghcr-login
+        if: ${{ steps.ghcr-credentials.outcome == 'success' }}
         uses: docker/login-action@v1
         with:
           registry: ghcr.io
           username: ${{ secrets.GHCR_USERNAME }}
           password: ${{ secrets.GHCR_TOKEN }}
+      - name: Check for Docker Hub credentials (secrets)
+        id: docker-hub-credentials
+        run: if [ ${{ secrets.DOCKER_HUB_USERNAME == null || secrets.DOCKER_HUB_TOKEN == null }} = true ]; then exit 1; fi
+        continue-on-error: true
       - name: Login to Docker Hub
+        id: docker-hub-login
+        if: ${{ steps.docker-hub-credentials.outcome == 'success' }}
         uses: docker/login-action@v1
         with:
           username: ${{ secrets.DOCKER_HUB_USERNAME }}
@@ -80,14 +92,22 @@ jobs:
           tags: |
             docker.io/${{ steps.vars.outputs.docker-hub-namespace }}/${{ steps.vars.outputs.candidate-tag }}
           cache-from: type=registry,ref=docker.io/${{ steps.vars.outputs.docker-hub-namespace }}/${{ env.cache-repository-name }}:dev
-          cache-to: type=registry,ref=docker.io/${{ steps.vars.outputs.docker-hub-namespace }}/${{ env.cache-repository-name }}:${{ matrix.target }},mode=max
-          push: true
+          cache-to: ${{ (steps.docker-hub-login.outcome == 'success') && format('type=registry,ref=docker.io/{0}/{1}:{2},mode=max', steps.vars.outputs.docker-hub-namespace, env.cache-repository-name, matrix.target) || null }}
+          push: ${{ steps.docker-hub-login.outcome == 'success' }}
       - name: Image digest
         if: ${{ !startsWith(github.ref, 'refs/tags') }}
         run: echo ${{ steps.build-push.outputs.digest }}
       - name: Release (pull candidate, tag, push)
         if: ${{ github.ref == steps.vars.outputs.tag-trigger-ref }}
         run: |
+          if [ "${{ steps.docker-hub-login.outcome }}" != "success" ]; then
+            echo "Docker Hub must be authenticated to perform a release!"
+            exit 1
+          fi
+          if [ "${{ steps.ghcr-login.outcome }}" != "success" ]; then
+            echo "GitHub Container Registry must be authenticated to perform a release!"
+            exit 1
+          fi
           docker pull docker.io/${{ steps.vars.outputs.docker-hub-namespace }}/${{ steps.vars.outputs.candidate-tag }}
           docker tag docker.io/${{ steps.vars.outputs.docker-hub-namespace }}/${{ steps.vars.outputs.candidate-tag }} docker.io/${{ steps.vars.outputs.docker-hub-namespace }}/${{ steps.vars.outputs.versions-tag }}
           docker tag docker.io/${{ steps.vars.outputs.docker-hub-namespace }}/${{ steps.vars.outputs.candidate-tag }} docker.io/${{ steps.vars.outputs.docker-hub-namespace }}/${{ steps.vars.outputs.latest-tag }}