Add launch script test

> 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 https://github.com/elixir-lsp/elixir-ls/issues/982#issuecomment-1729122176_