ci: run docker action only if docker username is set

.github/workflows/docker.yaml

@@ -10,6 +10,14 @@ on:
 jobs:
   build-and-push:
     runs-on: ubuntu-latest
+
+    # we need to copy the value to env,
+    # because the "if" at job level cannot access secrets directly
+    env:
+      DOCKER_USERNAME: ${{ secrets.DOCKER_USERNAME }}
+    # run the rest only if the env var is set and not empty
+    if: ${{ env.DOCKER_USERNAME != '' }}
+
     steps:
       - name: Checkout
         uses: actions/checkout@v4