A great number of self-hosted services out there offer easily deployable Docker images. Would be handy to have Inertia be able to do things like:

1. inertia init in arbitrary directory
2. In config, set build type to dockerhub and add a Dockerhub image name to pull
3. When inertia [REMOTE] up, pull the appropriate image and deploy it

What this gives Inertia:

• users can now see a cool third-party project and effortlessly deploy it with Inertia
• users now have the option to use Dockerhub's automated builds, and register a webhook to notify Inertia of any updates. This will require some work on More flexible webhook and git host support #133, however, to decouple our current webhook stuff from GitHub and make it more accepting of other webhooks.