[Docker] Review handling of volumes

We should re-investigate if we really need to to collectstatic on container startup. It seems we should use a VOLUME declaration instead, and ensure it is done in the correct order (the VOLUME declaration needs to be made after all data has been written to the source directory).

Also, we should check that the VOLUME declaration for AlekSIS' data honours the environment variables set media and backups (or probably declare two volumes for media and backups, even).