Add ShellCheck workflow

[szepeviktor]

Runs on PR-s.

From #471

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This is how to start locally.

git grep -Ile '^#!/bin/' | xargs -- shellcheck

... only 42 message in the whole repository

[szepeviktor]

• Hadolint runs shellcheck on Dockerfiles
• there is an urgent need for a shell script formatting tool

[jaydrogers]

Thanks again as always @szepeviktor! I will review and will keep you posted. I greatly appreciate you always helping me learn more 😃👍