Add stable release tag

[hardillb]

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)

Proposed changes

Add an extra tag that follows the minor version, e.g.

• version 3.0.x will be tagged as nodered/node-red:3.0
• version 3.1.x will be tagged as nodered/node-red:3.1

Checklist

• [x] I have read the contribution guidelines
• [ ] For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• [ ] I have run grunt to verify the unit tests pass
• [ ] I have added suitable unit tests to cover the new/changed functionality