add USER env var

[ric96]

android-x86 nougat build seems to fail with jack reporting an error for empty USER variable, this seems to be the solution...
haven't tested the solution yet, will update once the build has completed successfully.

[kylemanna]

This change won't make any changes because it will execute after the build is run (and fails).

Do you have any error messages or steps to re-produce?

[ric96]

Oh okay, so i am not using docker as a simple build bot, rather as an ubuntu environment that i log into to manually run aosp builds, as i am using arch Linux.
So for my usecase the pull request solution works for
‘‘‘
docker attach container
‘‘‘
But for
‘‘‘
docker exec container bash
‘‘‘
I need to add the same line to .bashrc

Also to reproduce the error follow build instructions from http://www.android-x86.org/getsourcecode
For nougat-x86 branch.

[robbyoconnor]

I also don't see this being necessary....the current works...and in most cases -- running as one-off containers is preferable...

[robbyoconnor]

I'm 👎 on this.

@ric96 -- I been using this and I run this like: docker-compose run --rm aosp -- in short -- I don't keep containers around...I just submitted #31 and #32 which adds gosu to the MM and master branch respectively -- so I think this PR should be closed as it doesn't add much.