Used 'flags.docker' in tests.

And some other minor style cleanups.
Exposed and used the Docker container name prefix.
Renamed variable to be more accurate.

Since we actually have a Docker::Container abstraction I
s/dockerContainer/dockerContainerizer/.
Refactored Docker::Container::pid() to return an Option.

Safer to check for 'None' than 0. Also fixed a compilation bug
comparing signed vs unsigned values with CHECK_NE.
Made Docker::ps/rm default arguments match CLI defaults.