Add BELUGA_DEPLOY_HOST

[duckbrain]

Problem

Currently DOCKER_HOST is abused to provide portainer and portainer-insuecure "protocols".

Docker, of course, doesn't understand these, and they're only useful for deploying. Usually times, users will want a different docker host to deploy to than to build.

Solution

BELUGA_DEPLOY_HOST will used to deploy when running beluga deploy. If not set, it will default to the value of DOCKER_HOST. It will replace the value of DOCKER_HOST when deploying, including setting DOCKER_HOST when running docker commands.

This will preserve backwards compatibility for users already using DOCKER_HOST, but in documentation, we can remove the sanctioning of abusing DOCKER_HOST.