Fix shellcheck CI job, fix/handle all shellcheck findings

[dbast]

This fixes the shellcheck CircleCI job:

• The existing job does nothing as the CI images does not support DIND (docker in docker), see error docker: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running? in https://app.circleci.com/pipelines/github/tilt-dev/tilt-extensions/1678/workflows/c1a75955-880d-408c-8188-cfe7651a9085/jobs/4065
• That is why the shellcheck circle orb is used instead.
• Even if that would work, it would still have always run green as the code in the Makefile target does not propagate all the errors.
• The Makefile target is updated so that the errors propagate to the final exit code ... for local bug fixing, without running circle ci.
• The Make target is further updated to use the official upstream container koalaman/shellcheck:stable, which is not 7 years old.

This PR additional fixes / handles all the previous not shown shellcheck findings, to get the noise down and the PR green -> CI becomes actionable feedback for new PRs.

[dbast]

yay. finally green after several iterations. Ready for review. thanks!