Document container image sources in README

[jaas666]

Added section on container image sources for Docker Hub and GHCR.

🗣 Description

Added section to let users know of the availability of a GHCR container

💭 Motivation and context

Closes issue #1254

🧪 Testing

N/A

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• I have read the CONTRIBUTING document.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Add a tag or create a release.