specifying max parallel jobs on push image

[pacostas]

Related issues

• https://github.com/paketo-community/ubi-base-stack/issues/32

Summary

Sets the parallel jobs that should run concurrently on push image workflow as the network cant handle it.

Use Cases

Checklist

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

[pacostas]

@mhdawson I havent tested the PR as it requires some amount of work in order to setup a dockerhub registry. But i think its safe to merge. Let me know if i should do otherwise.