Added option to config.yml to disable docker user namespace remapping for wings

[argetlam-coder]

Since wings does not work with docker user namespace remapping enabled, I added an option to the configuration that allows to disable user namespace remapping for wings. This way user namespace remapping does not have to be disabled globally. Other docker containers running on the server can still use user namespace remapping.

If you set the userns_mode option to host, wings will use the default docker namespace. By default, this option is "". Wings then uses the global settings.