refactor: move credentials checks into their own job

Lays the groundwork for splitting the `docker` job into `candidates` and `releases`.

PR: #41

.github/workflows/containers.yml

@@ -13,6 +13,20 @@ on:
 concurrency: ${{ github.workflow }}
     
 jobs:
+  credentials:
+    runs-on: ubuntu-latest
+    outputs:
+      ghcr: ${{ steps.ghcr.outcome == 'success' }}
+      docker-hub: ${{ steps.docker-hub.outcome == 'success' }}
+    steps:
+      - name: Docker Hub
+        id: docker-hub
+        run: if [ ${{ secrets.DOCKER_HUB_USERNAME == null || secrets.DOCKER_HUB_TOKEN == null }} = true ]; then exit 1; fi
+        continue-on-error: true
+      - name: GitHub Container Registry
+        id: ghcr
+        run: if [ ${{ secrets.GHCR_USERNAME == null || secrets.GHCR_TOKEN == null }} = true ]; then exit 1; fi
+        continue-on-error: true
   architectures:
     runs-on: ubuntu-latest
     outputs:
@@ -30,6 +44,7 @@ jobs:
             print('::set-output name=json::' + json.dumps(architectures))
   docker:
     needs:
+      - credentials
       - architectures
     runs-on: ubuntu-latest
     strategy:
@@ -40,24 +55,16 @@ jobs:
           - dev
           - build
     steps:
-      - 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' }}
+        if: ${{ needs.credentials.outputs.docker-hub == 'true' }}
         uses: docker/login-action@v1
         with:
           username: ${{ secrets.DOCKER_HUB_USERNAME }}
           password: ${{ secrets.DOCKER_HUB_TOKEN }}
-      - 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' }}
+        if: ${{ needs.credentials.outputs.ghcr == 'true' }}
         uses: docker/login-action@v1
         with:
           registry: ghcr.io