Update Run Image Names, Add Override Env Var, Update tests.

[BarDweller]

Summary

The Stack PR creating the Run Images has landed, so can replace the placeholder/best-guess run image names with the real ones now =) And then add tests, and provide an override to allow for escape hatch situations.

Use Cases

Building java apps with the extension =)

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).