Use well known DOCKER_HOST syntax for Docker publish

The main idea behind this PR is to replace `docker_server` and `docker_port` by `docker_host` directive, it follows the environment variable DOCKER_HOST format recognized by docker clients. Also export DOCKER_HOST instead of appending `-H` option to every docker call.

~~I included another bonus change, "docker push" is done per known tag to avoid pushing all existent tags everytime in case keep_build is true.~~ (#593 is better scoped for this change).  

Any feedback is welcome.