Create assets on first run in production mode

Currently, starting RDMS after rebuilding and reinstalling it from scratch in RAILS_ENV=production mode fails because assets are not created automatically. I'd prefer to put the required commands into the Dockerfile, but this doesn't work currently because the path used to store these assets is a Docker volume shared between multiple Docker containers, which are not available at build time of these container images/execution time of the corresponding Dockerfile. Therefore, for the time being, we execute the commands in the container's hyrax/docker-entrypoint.sh.

Closes #34 (closed)