misc: support/encourage usage of ShellCheck

My editor detected `.env` as shell script and thus automatically checked
it with ShellCheck. I would propose to make it a valid shell script that
complies with ShellCheck by default. To do this, we just need to disable
https://github.com/koalaman/shellcheck/wiki/SC2034 because the variables
are not used (in that file).

When you search for "docker-compose .env shellcheck SC2034" it turns out
that I am not the first one to do this :)