Allows overriding a user on the command line from the one specified in
the docker-compose.yml
The added tests verify that a specified user overrides a default
user in the docker-compose.yml file.
Based on commit f2f01e207b491866349db7168e3d48082d7abdda by @chmouel
Signed-off-by: Ian VanSchooten <ian@badgelabsllc.com>