CI: Improve how images workflow works in forks

Use the respository owner name rather than hardcoding 'mesonbuild'

Also don't try to push to docker if docker credentials aren't in secrets
(has to be written rather indirectly due to actions/runner#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'
+        run:  docker push ${{ github.repository_owner }}/{{ matrix.cfg.id }}