Image isn't automatically updated

[dannysauer]

This image was last updated on Dockerhub about a year ago, but the base alpine image was last updated a month ago. This I think is something which would be fairly straightforward to fix by using the specific alpine version in the dockerfile, implementing dependabot and a couple of actions to automerge updates, and having dockerhub rebuild on repo update (which may well already be the case).

PR shortly. :)