`containertool` should be able to set the default arguments passed to the entrypoint

[euanh]

Add an --args or --cmd command line parameter and add the value to the Cmd field in the configuration inherited from the base image.

[ebariaux]

Giving a go at implementing that, go the code working, want to add a test script.
I'll open a PR soon.

[ebariaux]

PR opened at: #155