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.