Determine a policy for adding dependencies to the GitHub Action

[dchiquito]

We recently had a request to add git as an installed package inside the GitHub Action image, because DANDI needs git available to determine some version information that is provided in an API endpoint that is being tested. I think this is an acceptable ask since git is such a ubiquitous tool, but it isn't feasible to update the image upstream for every consumer that needs a special package installed during test. Perhaps documentation describing how to build your own test image off of the base?