Support for: userns_mode

[cdrage]

No error / warning when using this key

[procrypt]

@cdrage This is libcompose issue.

$ 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' 

Issue created on libcompose docker/libcompose#444

[cdrage]

Closing this as I've updated the conversion document to reflect that this is not supported in Kubernetes and this key is also dropped in Docker Compose Version 3.