Combine Docker images, CPU+GPU?

[DanielKotik]

Do we want to have two separate Docker images (CPU and GPU specific)? It's possible to combine both Conda environments into one (big) image, which has the benefit of only one build-docker-image job and not two, almost identical ones for each target environment (from a container perspective, however, each container should cover only one single use case).
(This issue is a successor of #258.)