restore "container_prefix" config option and set default value of "user_container_name" to "{prefix}-{username}" for backward compatibility + add servername and prefix in namespace to generate the "user_container_name"
restore "container_prefix" config option and set default value of "user_container_name" to "{prefix}-{username}" for backward compatibility + add servername and prefix in namespace to generate the "user_container_name"

jupyter/dockerspawner,minrk/dockerspawner,jupyter/dockerspawner,minrk/dockerspawner