Add arg to set single label mode

[janeoa]

This is to be able to host builder and runner separately by passing an argument

[Narfinger]

I think we can do the following: Remove the single() from ContainerTypeIterator and just fill the remaining vec
Perhaps even a impl From<SingleLabelMode> for ContainerTypeIterator that is basically just your fn container_types.
I also wonder if the SingleLabelMode should have a different name. Something like container mode so that the both case is better included.