werf tag command currently not reimplemented from Ruby.
When Werf builds images, the result is a build cache — a set of image layers. To create a final image and assign a specified docker tag (see more about image naming here) use
werf tag command.
Build images with build commands before assigning tags.
docker tag command, which simply creates an alias for the specified image,
werf tag produces a new image layer with a specified name, see image naming article for the details. The result of the executing
werf tag command is docker images created locally.