Build: use Exercism's base docker image of nim for test-runner

[ynfle]

This uses exercism/nim-docker-base:18a2d8619faf474ee7afc7ae499f111271eb4e12 to copy over the alpine version of nim for the test-runner because it is already built and doesn't have to be rebuilt on every CI test.

The build times are reduced from ~7.5-8.5 minutes to ~2.5-3.5 minutes (still twice as slow as on ubuntu itself)

The reason for the time reduction is that it takes ~5.5-6.5 minutes to compile the nim compiler which is not related to the test runner itself and can be (and is offloaded) to https://github.com/exercims/nim-docker-base. Which only has to compile the nim compiler once for every change to the Dockerfile as it changes rarely and when it does it's mostly due to dependency upgrades.

[ynfle]

@iHiD @ErikSchierboom is on vacation now (if I understand correctly) so am tagging you as code owner instead

[ynfle]

@ErikSchierboom I thought you were on vacation! Thanks for the approval

[ErikSchierboom]

@ynfle I'm doing reviews once a week :) But yes: I am officially on vacation.

[ynfle]

Nice!