Improvements on the test runner

[mpizenberg]

Just a few ideas I've had at some point and I don't want to forget next time we'll need to update this test runner.

• [ ] ~Use symlinks for node packages elm and elm-test instead of copy~.
• [x] Split container in two, a builder and a runner. This is called multi-staged builds.
• [x] Try find a solution to retrieve debug logs in the associated tests outputs.
• [x] Use jq instead of custom bash script for compilation errors, as in https://github.com/exercism/rust-test-runner/pull/11/commits/95e8d41fd32f8cf4773ee452e39e2fdc704a9842