Using a bash script instead of using a dockerfile for rebasing branches on builders

[pacostas]

Summary

This PR:

• Alters the rebase action to use bash script instead of bash script inside a dockerfile, as is totally unecessary

Use Cases

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).