Make pushes dependent on environment variables.

This means that forks can still deploy to either GAR or Dockerhub
depending on which credentials that have been configured with, and
skips any that are missing.

.github/workflows/deploy.yaml

@@ -28,26 +28,29 @@ jobs:
         with:
           images: |
             ${{ vars.DOCKERHUB_REPO }}
-            ${{ vars.GAR_LOCATION }}-docker.pkg.dev/${{ vars.GCP_PROJECT_ID }}/${{ vars.GAR_REPOSITORY}}/autograph-edge
+            ${{ vars.GCP_PROJECT_ID && format('{0}-docker.pkg.dev/{1}/{2}/autograph-edge', vars.GAR_LOCATION, vars.GCP_PROJECT_ID, vars.GAR_REPOSITORY) }}
           tags: |
             type=semver,pattern={{raw}}
             type=raw,value=latest,enable={{is_default_branch}}
 
       - id: gcp-auth
+        if: ${{ vars.GCP_PROJECT_ID }}
         uses: google-github-actions/auth@v2
         with:
           token_format: 'access_token'
           service_account: artifact-writer@${{ vars.GCP_PROJECT_ID}}.iam.gserviceaccount.com
           workload_identity_provider: ${{ vars.GCPV2_GITHUB_WORKLOAD_IDENTITY_PROVIDER }}
 
       - name: Login to Google Artifact Registry
+        if: ${{ vars.GCP_PROJECT_ID }}
         uses: docker/login-action@v3
         with:
           registry: ${{ vars.GAR_LOCATION }}-docker.pkg.dev
           username: oauth2accesstoken
           password: ${{ steps.gcp-auth.outputs.access_token }}
 
       - name: Login to Dockerhub
+        if: ${{ vars.DOCKERHUB_REPO }}
         uses: docker/login-action@v3
         with:
           username: ${{ vars.DOCKERHUB_USERNAME }}