Add options to override CMD for container

[thenewguy]

It would be helpful to specify the command that should be used for running the container.

This is the example image from my use case

The container exposes several commands via one docker image for testing purposes - in this particular case - I want to run ippeveprinter but there is no way to add it.

I believe it should be simply adding the option and passing it along at https://github.com/tox-dev/tox-docker/blob/ade52c44dd9199a666c2525ead471ab1323932a1/tox_docker/init.py#L303