Adding note about shell for Container Jobs

[joshjohanning]

Why:

The default shell for container jobs is sh vs. bash. This can cause run scripts to fail if using bashisms (for example, if statements with double brackets).

Calling this out in the reusable for a job container.

Reference:
actions/runner#1533 (comment), which references: actions/runner#353 (comment)

What's being changed (if available, include any code snippets, screenshots, or gifs):

Adding the content in the blue note:

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Writer impact (This section is for GitHub staff members only):

• This pull request impacts the contribution experience
  • I have added the 'writer impact' label
  • I have added a description and/or a video demo of the changes below (e.g. a "before and after video")

[cmwilson21]

👋 @joshjohanning Thanks so much for opening a PR! I'll get this triaged for review ⚡

[lucascosti]

Thanks a lot for this, @joshjohanning! I'll get it merged.

[github-actions]

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