Update and document the new official docker image

[sandreas]

Todo:

• [x] Bump tone version in https://github.com/sandreas/dockerhub-builds/tree/main/m4b-tool/latest
• [x] Test and upload new latest pre-release (with workaround for tone being removed)
• [x] Manually update the latest official m4b-tool docker image
• [x] Update Dockerfile in m4b-tool repository
• [ ] Publish anouncement (see below)

There is a new "official" (still experimental) docker multiarch image and SHOULD work on x64, and Raspberry platforms. It is completely handcrafted and available for the following archs:

• linux/arm/v6
• linux/arm/v7
• linux/arm64
• linux/amd64

docker pull sandreas/m4b-tool:latest

In the future, it will use tagging based on my new project tone, which SHOULD be a huge improvement over the existing mp4v2 and ffmpeg strategies.

I need your help: Try out the new docker image and give feedback, so that we can get to a production ready release as soon as possible.