Fix docker environment for search service

[humphd]

We need a few tweaks to our environment for the Search service. The search service needs to be able to talk to Elasticserach via the Docker network, so the domain name has to be the elasticsearch container vs. localhost, or it can't reach outside of its own container. Also, we need to do this in all configurations, not just in dev or production, so I've moved it to docker-compose.yml. Finally, we need to expose the POSTS_URL to the environment of the search service, so that it can construct URLs.