Trigger build to Docker Hub

Summary:
When a code repository is updated, we need to also update the Docker
image.

Fixes T959.

Test Plan:

• fire a GitHub push event
• check it triggers a new build on Docker Hub

…
{
  "image": "nasqueron/notifications",
  "action": "TriggerDockerHubBuildAction",
  "error": null
},
…

Reviewers: dereckson

Subscribers: alken-orin

Maniphest Tasks: T959

Differential Revision: https://devcentral.nasqueron.org/D636