patch (docker): uncomplicate volumes

volumes need not be mounted; declaration suffices

changes

1. docker-compose.yml

   remove all volumes mounts

2. Dockerfile

   declare volumes at end

   Changing the volume from within the Dockerfile: If any build steps change the data within the volume after it has been declared, those changes will be discarded.

   source

3. README.md

   • remove mounts from instructions
   • compose installation is a lot simpler now and default on Linux with convenience script
     • documented at https://docs.docker.com/compose/install/#install-compose-on-linux-systems