Don't run CircleCI docker build for pull requests

[Trikolon]

It fails and we don't really need it other than for pushes. E.g. see #6
@ahoneiser do you agree, or is there a reason why we need it to run for PRs too?

[Trikolon]

It looks like it's failing if the PR author or person who merges is doesn't grant CircleCI access to the repository. This is done by logging into CircleCI via GitHub. Seems like an o.k. requirement for Mozillians but doesn't really work for external contributors.
We should consider:

1. Splitting up the repos and put the syncing script into a separate one.
2. Not running the CircleCI steps for PRs.

I'm in favor of (1).