Other hooks process json then execute commands. The docker hook
makes socket API calls via the docker-py module. For this reason
a different approach has been taken with testing this hook:
- symlink hook-docker.py to the tests package as hook_docker.py
- in the test, mock out the docker import and run main() directly
Change-Id: Ia069b2cc0db71952732dea310a8986b73981b656