Support specifying a user under which to run the container

[nhavens]

I'd like to be able to use tox-docker both to test via Jenkins as well as to test on my local machine. tox will run as a different user in each of these environments. To get the correct file permissions on any files created on volume mounts, I'd like to be able to specify the user under which to run the container. Without this, when I volume mount the directory from which tox runs and any files get created by the container with root ownership (I'm running as root in the container), the next Jenkins job's deleteDir() will fail with permission denied errors.

[dcrosta]

tox-docker 4.1.0 added support to build & use a Dockerfile in the repo. Does the USER command in a Dockerfile work for your needs here?