After [a discussion][1] with rje6459 on the networktocode Slack
in the #netbox-docker channel we've come up with a small improvement
to those that regularly have to build netbox docker locally.

It fetches the "requirements.txt" from the desired branch before
downloading netbox itself.

[1]: https://networktocode.slack.com/archives/CD23LP8BC/p1558537080023200