gltestserver needs a diet

The recently introduced gl-testserver is run from a docker image that
contains all the necessary dependencies, and some more. The image
weighs in at over 9GiB, which even developing locally is a bit
much. It means that the developer disk is pointlessly being filled,
but also that a build and download of the docker image is a lengthy
process.

Putting it on a diet to weigh less, and come with fewer optional
dependencies is likely a good idea. These are the common threads that
we might want to investigate:

- [ ] Rust compilation artifacts should be kept in a builder image, and
selectively copied over.

Start by locating how gl-testserver builds and runs its Docker image, then measure which dependencies and Rust compilation artifacts are included. Compare the image contents and size before and after separating builder artifacts and removing optional dependencies; done means the server still works with a substantially smaller image.

Written by the indexing model from the issue text.