Allow Dockerfiles in build field

A potential stop-gap solution for this: https://github.com/docker/docker/issues/2112.  While a proper solution is being developed, to allow one Dockerfile to handle multiple targets ( or whatever solution in decided upon ), this code simply copies the Dockerfile that is being built to a file named Dockerfile, then deletes it after the build is done. If there is already a Dockerfile in that directory, it is copied to a backup location and restored after the build. If a path is specified, then the behavior is the same as previously.

Allows for a fig.yml that looks like this:

<pre>db:
  image: postgres
  ports:
    - "5432"
dev:
  build: dev.Dockerfile
  command: bundle exec rails server
  ports:
    - "3000:3000"
  links:
    - db
prod:
  build: prod.Dockerfile
  command: bundle exec rails server
  ports:
    - "3001:3000"
  links:
    - db
test:
  build: test.Dockerfile
  command: bundle exec rails server
  ports:
    - "3002:3000"
  links:
    - db
</pre>

Signed-off-by: Adam Ard <aard@instructure.com>