Shell script linter #63

We should set up a shell script linter on the CI to reduce bugs:

• add a new CI job (it should follow the pattern, separate yml file ...)
• use shell check as linter
• make sure our existing shell files are linted (fixing the issues might require a follow up PR, depending how much there are)

We have something similar in the Docker CLI:

• https://github.com/docker/cli/blob/master/.github/workflows/validate.yml#L35