Improvements to testing, more usage documentation and bumping hadolint.
Commits:
- 693448f fix: shellcheck source for test inclusions
- bc3442d feat: test multiple dockerfiles as input
- 4ba63d1 doc: add USAGE.md
- a1fff55 feat: hadolint 2.7.0
- 05d7039 feat: switch to wget and unify execution
- d540198 feat: test action on pr
- bc00c6b fix: typo in action test
- 4afe5ef fix: repo path url
- 23ff87f feat: check github action syntax with actionlint