Further test runner work to simplify

[joshgoebel]

Docker Hub:

• [ ] javascript_test_runner Needs: https://github.com/exercism/javascript-test-runner/pull/17
• [x] csharp_test_runner (needs nim webhook added)

Nim tooling:

• [x] Build common Nim local web hook tooling and use that in single Dockerfiles to avoid the whole "-server" vs "-base" naming nightmare.
• [x] Get nim hook to build automagically
• [x] Migrate to nim hook, test with whole stack.