CI: Prevent docker release when hopeless

[why]
On forks for example they might have a docker repo associated or not.
The release process is run in any case and will fail in that cases.

[how]
Check that at least the secret is known.

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

.github/workflows/docker-release.yml

@@ -17,6 +17,7 @@ jobs:
   publish-image:
     name: Publish image
     runs-on: ubuntu-latest
+    if: ${{ secrets.DOCKER_USER }}
     steps:
       - uses: actions/checkout@v3
       - name: Build image