[FEATURE] Provide environment variable to resolve relative paths in config file

[fsimmend]

It would be awesome to have an environment variable set by k3d to resolve the location of the config file within the config file. With that environment variable you could mount host directories relative to the config file. Since there is only one possible config file this should be possible.

The current workaround is to use ${PWD} in the config file, but that requires you to jump into that directory.