Updates to CI

@berkgeveci @NAThompson I decided not to pull the VTK-m build out of the docker images (re: discussion in !32 (merged)), but I did simplify things. There's only a single place where the commit hash for VTK-m needs to be updated, which is in .gitlab/ci/docker/update-images.sh and that script will handle making the necessary change to .gitlab-ci.yml along with building the docker images and pushing them to docker hub. Although in order for the push to docker hub to be successful, you have to have your docker hub username added to the Kitware repository. I have access, not sure if Berk does, but since it's not something that will happen that frequently, I can just run the script whenever we're wanting to change it again.

Also VTK-m is now built in debug mode, and for the ASAN build, it's also built with the same sanitizer options.