docker/fig — pull request #445 by suzaku (2014-08-26 03:37:18+00:00)
Title: 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.