Support for userns_mode

[procrypt]

If my docker-compose file containes userns_mode directive I get this error.

$ libcompose up
WARN[0000] Note: This is an experimental alternate implementation of the Compose CLI (https://github.com/docker/compose) 
ERRO[0000] Could not parse config for project komposefiles : Unsupported config option for web service: 'userns_mode' 
FATA[0000] Failed to read project: Unsupported config option for web service: 'userns_mode'