Backup/restore users and configuration

There is no obvious way to modify or specify an alternative configuration file and every time the docker container is restarted a new configuration file is generated changing the admin password.  This makes it difficult to configure a build server for automated publishing.  I have changed the start script to only generate a new configuration file if one does not already exist. 

Since users and configuration are stored on in /opt/sinopia rather than /opt/sinopia/storage I changed the recommended volume mount point to /opt/sinopia and added instructions to the README for modifying the configuration and performing backups and restores.