Enhancement: pre-built docker image for mage

Describe the feature
Provide a pre-bulit docker image for mage, so it can be used without installing/cloning/building/etc.

The docker contain would need go installed already, so really it's just a go docker container where someone team go install GitHub.com/magefile/mage@latest ... And that doesn't sound complicated enough for someone else to need my help. And not something I want to have to maintain.