Add launch script test

[lukaszsamson]

          > Nice. I wonder if we could add CI tests based on that dockerfile.

I thought about that, too - and i guess i would have done it but i couldn't manage to run the language_server.sh using fish. But yeah, i guess it is possible to have an automated test for it.

Where would you like to have that test, in a /test-directory?

Not sure if I understand that. VSCode devcontainers already work with https://github.com/elixir-lsp/vscode-elixir-ls

Yeah - sorry - i meant i could add a devcontainer setup for this project (i experimented a lot with devcontainers in the recent two years) which would allow an developer to onboard just having docker desktop and VSCode but comes at the cost of one additional directory (.devcontainer) in the project.

Originally posted by @florianb in #982 (comment)