Install git & git-lfs in all base containers

A common use for the igwn/base containers is to run CI jobs on git.ligo.org. Typically these jobs will need to check out a Git repository, and then run a CI script to perform some actions on the repository contents.

In order for the GitLab runners to check out a repository, git must be installed in the container used to run the CI job. This is because GitLab checks out the repository before running the CI script, so there's no opportunity for the CI script to install git first. In addition, for any repository that uses Git-LFS, git-lfs must also be installed and configured in the container.

The igwn/base containers do not currently require git/git-lfs to be installed. Therefore, any CI pipeline which requires a Git repository checkout cannot use the igwn/base containers directly, and instead would need to create a new container derived from igwn/base which has git/git-lfs installed.

Since running CI jobs on a Git repository is a common enough use case, it seems reasonable to have git/git-lfs installed into the igwn/base containers. That way, anyone can use those containers (or any containers derived from them) and repository checkouts on GitLab runners will always work correctly. It seems a reasonable expectation that repository checkouts in CI jobs should "just work", and it seems reasonable to have this set up correctly in a low-level container (i.e. igwn/base) rather than relying on downstream container users to figure this out for themselves.

A downside might be the additional size this adds to the igwn/base containers. From some testing, installing git/git-lfs into an igwn/base container (where neither was previously installed) typically adds a ~100 MB layer; I don't know if that's of concern or not. On the other hand, this size would then be saved from any downstream container based on an igwn/base container, since those containers no longer need to install git/git-lfs themselves. Since many containers that work with repository checkouts would need git/git-lfs installed, this may even result in an overall space saving.