feat: enable to push to docker repo set in settings

If nothing is set in settings, we'd push to `ghostfolio/ghostfolio` as
before. This commit will enable easier maintenance of forks by people
outside of `ghostfolio` org.

Signed-off-by: Anatoly Popov <me@aensidhe.ru>

.github/workflows/docker-image.yml

@@ -19,7 +19,7 @@ jobs:
         id: meta
         uses: docker/metadata-action@v4
         with:
-          images: ghostfolio/ghostfolio
+          images: ${{ vars.DOCKER_REPOSITORY || 'ghostfolio/ghostfolio' }}
           tags: |
             type=semver,pattern={{major}}
             type=semver,pattern={{version}}