Deploy application via docker image

[richard-cox]

• this should be pretty straight forward
• we should check the feature flag diego_docker is enabled (should also consider eirini flags)

[richard-cox]

Need to decide if this process should be either

1. A new deployment source which provides an app name and docker image

   • Requires some medium changes in the backend
   • Attempting to push again after failure would result in route taken errors

2. As an option in the overrides screen override value

   • User would still need to provide source (github, upload, etc)
   • docker image is an additional option in the overrides page

In both cases we'd supply the --docker-image option to the cf push command