Support non-1000 uid/gid with Docker 1.10

[itamarst]

Not all users are uid 1000, as your docs note. In latest version of Docker there is feature that would allow specifying which uid to use.

Specifically Docker 1.10 supports "build args" (https://docs.docker.com/engine/reference/builder/#arg). At work we've built a Dockerfile Go dev environment that allows setting custom uid and gid for the Docker containers using these; I assume devstep could do the same.

[fgrehm]

Its on my radar, not sure when I'll get to it though

[lcmen]

I think it could be vsolved by building the devstep image locally so you can pass the build argument.

Alternatively you could pass it to run command and fix permission during container startup.

For now I've just forked the repo and changed the id to 501:21 (OS X), seems to be working.