* workflow: build and test pushes and pull_requests to master ; deploy image to GHCR
* improved tiles verification (*)
- Export the tiles as an artifact before verifying.
- Only empty.png is verified via checksum.
- Verify that all tiles are image/png files.
- Verify that all tiles are different from another.
* import and start the test server with automatic updates
* DockerHub deployment (*)
if secrets.DOCKERHUB_USERNAME and secrets.DOCKERHUB_PASSWORD are non-empty
