How not to do it:
When building an image, you could also tag it this way.
docker build -t ubuntu:14.04 .
Then you build it again with another tag:
docker build -t ubuntu:latest .
If your Dockerfile makes good use of the cache, the same image should come out, and it effectively does the same as retagging the same image. If you do docker images then you will see that they have the same ID.
There's probably a case where this goes wrong though... But like @david-braun said, you can't create tags with Dockerfiles themselves, just with the docker command.