Docker: stop publishing "latest" image tag

[reinkrul]

We publish a latest Docker tag that points to the latest release (e.g. v5.4.5). In my opinion, we should stop publishing latest and retract the existing latest tag from Docker hub.

• People should not use the "latest release" because it's safe (since it's not the latest master tag): a major version can break deployments, so it's unsafe to use in production.
• When talking about Nuts network, "stable" is a network name. In ReadTheDocs, "stable" refers to the documentation of the latest released version. These are different concepts with the same name.
• When talking about Docker image tags, "latest" means the most recent release, but in ReadTheDocs "latest" is the latest build from master. More confusion.
• "latest" docker tag sometimes goes awry, when builds race to publish their images; it's conceived as the "highest" released version, but sometimes it's the latest published version.

Instead, people should find the newest major release and use that one (that's what I always do when deploying new Docker containers), or pinpoint a minor or patch version, depending on company policy. Minor versions don't break functionality, so automatic upgrade should be safe there.

We can probably keep master.

[reinkrul]

Decided against it

[woutslakhorst]

latest tag is used in the getting started for devs