gltestserver needs a diet

[cdecker]

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.