Better document and revise actor parameters

[heikomuller]

Is your feature request related to a problem? Please describe.
It is unclear how to define default values for parameters of type actor. There is no documentation available. The current implementation seems to suggest a tuple consisting of the keyword container and a dictionary with elements image and commands. For example, it is not clear why that dictionary doesn't have the element environment instead of image?

Describe the solution you'd like
Better document the actor parameter and review the current implementation.