Issue #104: Update latest image on Docker Hub only if it is a stable release

[sushantmimani]

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• Tests (if applicable)
• Documentation (if applicable)
• Changelog entry
• A full explanation here in the PR description of the work done

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Tests
• Other

Backward Compatibility

Is this change backward compatible with the most recently released version? Does it introduce changes which might change the user experience in any way? Does it alter the API in any way?

• Yes (backward compatible)
• No (breaking changes)

Issue Linking

Fixes #104

What's new?

• The latest image on Docker Hub will be updated only when we make a stable release.
• macOS-11 is going to be used as macOS-latest and that doesn't support pypy-3.6, so upgrade to pypy-3.7
• Use latest version of the setup-python action