Avoid repeating build of the same image

Since there can be only one Dockerfile in a build context, we often
end up reusing one image in our project for multiple purposes.

In that case, the `fig.yml` would look like:

```yaml
service1:
    build: .
    command: aaa
service2:
    build: .
    command: bbb
service3:
    build: .
    command: ccc
test:
    build: .
    entrypoint: make
    command: test
    link:
        - service1:service1
        - service2:service2
        - service3:service3
```

Now when we use `fig build`, `fig` tries to build all three of them.

This commit aims to avoid unnecessary `docker build`s, if an image for
the build path has already been built, we just skip that service.

Signed-off-by: Xuecong Liao <satorulogic@gmail.com>