Fix Dockerfile build issue

[ErikSchierboom]

I've found that the Dockerfile does not build on my machine, nor does it build on the build server: https://github.com/exercism/erlang-test-runner/actions/runs/813744510

Converting the aliases to lowercase fixes this.

[ErikSchierboom]

@NobbZ We've merged this to try and fix the build issue, as otherwise the test runner can't be deployed.

[ErikSchierboom]

And I can confirm that the Docker building part now works: https://github.com/exercism/erlang-test-runner/actions/runs/816844440

If I run the test runner on the hello-world exercise at exercism.lol though, I get an error. This is in the logs:

Exercise: hello-world
Solution: /mnt/exercism-iteration/
Output:   /mnt/exercism-iteration/
escript: exception error: bad argument
  in function  binary_to_existing_atom/2
     called as binary_to_existing_atom(<<"example_tests">>,utf8)
  in call from etr_runner:get_test_module_from/1 (/app/src/etr_runner.erl, line 27)
  in call from etr_runner:run/1 (/app/src/etr_runner.erl, line 6)
  in call from erlang_test_runner:main/1 (/app/src/erlang_test_runner.erl, line 15)
  in call from escript:run/2 (escript.erl, line 758)
  in call from escript:start/1 (escript.erl, line 277)
  in call from init:start_em/1 
  in call from init:do_boot/3 

Any idea @NobbZ ?

[NobbZ]

Probably a misnamed file. You provided example.erl, which contains the example module, so it searches for the example_tests module, but that does not exist as the tests for hello_world live in hello_world_tests.

I made some very strict assumptions about module names for the practice exercises, as that is how the tests are written in general.

It is exactly the same issue as months ago: exercism/v3-launch#4 (comment)

[ErikSchierboom]

@NobbZ I finally found out why this failed. The example implementation did not define the correct module. It should have been hello_world instead. I can confirm that with the right module name, it works!