In previous conversations I think we agreed that the ability to tag containers would be nice, but there was no compelling reason to add it to the core. Especially with globally unique IDs, it's super easy for a user to store all the metadata he needs himself: users, applications, services, versions, source repository, source tag, whatever.

However there is one thing that is only possible in the core: atomic operations on a set of containers matching certain tags. In the future that might be a necessary feature, for a number of reasons:

a) Performance (to avoid running 200 duplicate commands for 200 containers)
b) Reliability (eg. less moving parts when coordinating many dockers)
c) Ease of development

I don't have a set opinion, but wanted to write this down for later discussion.