add authentication for private docker repositories

[sim-san]

Feature description:

Add the env varibales for authentication against a private docker repositories.

Problem and motivation:

We customize the workspace image by installing internal liabries. This image is save in a private and secured docker repository. Right now I have to manually pull new images after running docker login.

Is this something you're interested in working on?

I could not immediately find a place in the code where this change could easily be made.