Add shell script linting to CI

[grayside]

Using the shellcheck utility we can ensure shell scripts are well-formed, consistent, and without silent failures.

shellcheck can be run via Docker.

Note: shellcheck is used by hadolint to scan the shell statements inside Dockerfiles. hadolint is currently under consideration as a distinct Dockerfile linting check in #1374

[grayside]

Assigning to fhinkel for consideration.

[fhinkel]

Closing this as there are more pressing issues in this repo. Feel free to re-open if you want to work on it.