use skopeo login for docker login

[sophiewigmore]

Summary

The create-stack workflow is failing on the docker login step. This changes the login to use skopeo login like we do in paketo-buildpacks/github-config.

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)
• [ ] 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).

[sophiewigmore]

Closing because skopeo login isn't the solution to this problem. The problem is with the fact that: Note: With the exception of GITHUB_TOKEN, secrets are not passed to the runner when a workflow is triggered from a forked repository. (from https://docs.github.com/en/actions/reference/encrypted-secrets#using-encrypted-secrets-in-a-workflow), and the PR that broke this workflow is from a fork.