CI: Don't try to push docker image in a forked repository

The 'build images' workflow runs on a schedule, so will run (and fail)
weekly in every fork.

Don't try to push to docker if docker credentials aren't in repository
secrets.

(A test for that has to be written rather indirectly due to
https://github.com/actions/runner/issues/520)

.github/workflows/images.yml

@@ -21,6 +21,8 @@ on:
 
 jobs:
   build:
+    env:
+      HAS_DOCKER_CREDENTIALS: ${{ secrets.DOCKER_PASSWORD != '' }}
     name: ${{ matrix.cfg.name }}
     runs-on: ubuntu-latest
     strategy:
@@ -38,7 +40,7 @@ jobs:
 
       # Login to dockerhub
       - name: Docker login
-        if:   github.event_name == 'push' || github.event_name == 'schedule'
+        if:   (github.event_name == 'push' || github.event_name == 'schedule') && env.HAS_DOCKER_CREDENTIALS == 'true'
         uses: azure/docker-login@v1
         with:
           username: ${{ secrets.DOCKER_USERNAME }}
@@ -52,5 +54,5 @@ jobs:
 
       # Publish
       - name: Push the ${{ matrix.cfg.id }} image
-        if:   github.event_name == 'push' || github.event_name == 'schedule'
         run:  docker push mesonbuild/${{ matrix.cfg.id }}
+        if:   (github.event_name == 'push' || github.event_name == 'schedule') && env.HAS_DOCKER_CREDENTIALS == 'true'