Maintain Docker Environments for Doc Builds

[WillAyd]

Right now the docker environment for building the documentation is destroyed when make doc finishes. Would be better I think to maintain that environment in case of issues that need to be debugged, and leave it to a separate make clean or something similar to actually delete the environment

ref pandas-dev/pandas#29374 (comment)

[TomAugspurger]

Seems reasonable. Just need to remove the --rm.