Dockerfiles + CI to build images

[justinlaughlin]

It would be nice to have dockerfiles / pre-built images, at least for the most common setups (e.g. the smaller test matrix used in the fast CI option). Eventually these could be hosted on mfem/containers but for now because of the relative stability of mfem vs pymfem I suggest we simply host it here, using something similar to the mfem container CI.