Contributions welcome - just raise a PR and make sure the tests pass!
GOV.UK Docker is versioned in order to notify users about important changes, where they may need to take action. Since we use the repo itself for distributing GOV.UK Docker, for many PRs it's unlikely you will need to change the version.
A consequence of this is that GOV.UK Docker does not (need to) follow semantic versioning.
Any change to the version should be accompanied with an explanation in the CHANGELOG. When the version changes, a user running the govuk-docker CLI will see a one-time notification, which includes the new CHANGELOG entry.