updated xcube GH workflow to build docker image only during releases

[TejasMorbagal]

[Description of PR]

This PR updates xcube GH workflow to build docker image only during releases and disables docker image build and push for each commit to master

Checklist:

• ~[ ] Add unit tests and/or doctests in docstrings~
• ~[ ] Add docstrings and API docs for any new/modified user-facing classes and functions~
• ~[ ] New/modified features documented in docs/source/*~
• [x] Changes documented in CHANGES.md
• [x] GitHub CI passes
• [x] AppVeyor CI passes
• [x] Test coverage remains or increases (target 100%)