docker v2: add tag as a content unit type
With the Pulp 2.8 updates to support Docker v2, it has enhanced the implementation for docker tags to become a content unit (similar to manifest, rpm, package group...etc).
This issue is to add the logic to support the new type.
fixes #13512 - docker v2 - pulp 2.8 - add support for tag content unit
With the pulp 2.8 support for docker v2, the docker tag implementation
has been updated to be a content type, similar to manifest, rpm...etc.
This commit adds that unit.