Ability to add extra tag builds

When there is a container A that container B depends on, in B's Dockerfile I reference it with the command `FROM myname/A`. Unfortunately fig only tags with docker_A, but I don't want to use that in my Dockerfile. Something like the following would be good

```
A:
  build: A
  tags:
    - myname/A
B:
  image: myname/A
```
