Add an action to lint shell scripts and Dockerfiles

[zb140]

Added an action to run linters for shell scripts and Dockerfiles. It will run on any PR. If there's a lint issue in one of the files being modified, it will add a review pointing out the specific line and error. If it finds errors in files not in the PR, those will get noted under the "Checks" tab. None of this blocks merging the PR anyway, but it should make it much easier to spot potential problems.

[zb140]

Note that I have not yet gone through and fixed all the existing lint issues; my plan is to do that after merging #76, since there's changes to pretty much all the files that would be affected by this.

[Sparticuz]

While we're at it...

https://github.com/dotenv-linter/action-dotenv-linter

https://github.com/marketplace/actions/yaml-lint

[zb140]

While we're at it...

https://github.com/dotenv-linter/action-dotenv-linter

https://github.com/marketplace/actions/yaml-lint

Good idea! I've added them.

[Sparticuz]

Lgtm