CI: Improve how images workflow works in forks

Use the respository owner name rather than hardcoding 'mesonbuild'

Don't run this workflow if docker credentials aren't in secrets
(has to be written rather indirectly due to actions/runner#520)

.github/workflows/images.yml

@@ -21,6 +21,9 @@ on:
 
 jobs:
   build:
+    env:
+      HAS_DOCKER_CREDENTIALS: ${{ secrets.DOCKER_PASSWORD != '' }}
+    if: ${{ env.HAS_DOCKER_CREDENTIALS }}
     name: ${{ matrix.cfg.name }}
     runs-on: ubuntu-latest
     strategy:
@@ -53,4 +56,4 @@ 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 }}
+        run:  docker push ${{ github.repository_owner }}/{{ matrix.cfg.id }}