add support for manually assigning the docker image name

[missinglink]

I recently merged pelias/docker#274, unfortunately the Docker image name and tags are not correctly assigned to the Elasticsearch image because they are being detected from the pelias/docker repo.

Fortunately the pelias/docker repo has no need to be Dockerized itself, but it would be nice if we could make some changes to the scripts in this repo to allow a workflow such as this.