feat: add GitHub Actions workflow for Docker image build and publish to GHCR and Docker Hub.

.github/workflows/docker-publish.yml

@@ -13,6 +13,9 @@ env:
   REGISTRY: ghcr.io
   # github.repository as <account>/<repo>
   IMAGE_NAME: ${{ github.repository }}
+  # Map secrets to env for availability in 'if' conditions
+  DOCKER_USERNAME: ${{ secrets.DOCKER_USERNAME }}
+  DOCKER_PASSWORD: ${{ secrets.DOCKER_PASSWORD }}
 
 jobs:
   build:
@@ -41,7 +44,7 @@ jobs:
       
       # Login against Docker Hub (Optional fallback if secrets exist)
       - name: Log into Docker Hub
-        if: github.event_name != 'pull_request' && secrets.DOCKER_USERNAME != ''
+        if: github.event_name != 'pull_request' && env.DOCKER_USERNAME != ''
         uses: docker/login-action@v3
         with:
           registry: docker.io