Split docker images on image name instead of by tag

Right now, frontend and backend are different tags. It would be better to have two different images for frontend and backend, e.g.

```yaml
# docker-compose.yml
services:
  frontend:
    image: ghcr.io/metalab/metadrinks-frontend:latest
    # ...
  backend:
    image: ghcr.io/metalab/metadrinks-backend:latest
    # ...
```