Allow array of strings to be passed to entrypoint

[peacememories]

The docker-compose (and docker) spec allows (and even prefers, from what I can tell) users to supply the entrypoint as a list of command and arguments. It would be nice to have this ability in arion as well.

[roberth]

Absolutely. I would expect this to be as simple as changing a str to either str (listOf str), and change a test to use it that way. I see little use in testing the old way.