Introduce stable tag for docker images

[LesnyRumcajs]

Issue summary
We are currently using the moving tag latest, which may be confusing. See an interesting article dedicated to this problem here.

It'd be beneficial to change the behaviour a bit:

• latest shall denote the latest stable release

• edge shall denote the latest development build (latest main build)

• introduce edge tag to the Docker workflows

• update the documentation

• update the forest-iac infra so that it uses the edge tag for tests

Other information and links

[hanabi1224]

How about using latest for the latest stable release and dev for rolling builds?

[LesnyRumcajs]

I'd go with edge and latest as described in
https://devops.pingidentity.com/docker-images/releaseTags/#edge
https://devops.pingidentity.com/docker-images/releaseTags/#latest

Also this approach seems to be supported by docker meta action.
https://github.com/docker/metadata-action#typeedge

Sounds good?