Beside providing tox as a Python package it could be helpful to also provide an "official" Docker image that's set up with correct Pythons and tox installed. This came out of #943. Any version of this needs to:

• automate docker image update as part of the release process (Azure Pipelines has docker support),
• add image files,
• add tests (Azure Pipelines has docker support).

It's to be determined if this should leave as part of this repository; or should it instead be a separate project.