Change DOCKER_ENDPOINT to DOCKER_HOST

[dhobbs]

Boot2Docker (https://docs.docker.com/installation/mac/) uses the environment variable DOCKER_HOST to point to Docker:

echo $DOCKER_HOST
tcp://192.168.59.103:2375

It would be great if uberimage also looked for DOCKER_HOST (either in addition to, or instead of DOCKER_ENDPOINT). That way it would Just Work for boot2docker users.

[hugoduncan]

Seems like a good idea. I think it might be advisable to user DOCKER_ENDPOINT if set, and fall back to DOCKER_HOST, just in case DOCKER_HOST points to a unix socket endpoint, and an http endpoint is being used just for lein-uberimage. Does that make sense?