allow custom hostnames setting for db, elastic, rabbit with envvar

[nicolasochem]

You can not always assume the hostname of the remote services.

For example, in tezos-k8s, I am running everything in a pod, so
I need to set everything to localhost.

In order to not disrupt any existing workflow, we set these env
vars to standard values in the Dockerfile; they can be overridden
at runtime. See: https://stackoverflow.com/a/40334819/207209

[aopoltorzhicky]

Hi, thanx for pull. Seems like you skip dockerfile of others our services e.g. metrics, compiler etc. But I think it will be better use default env in docker-compose instead of adding layers in dockerfile. Something like that: key: ${ENVVAR:-defaultvalue}

[nicolasochem]

thanks @aopoltorzhicky , this is indeed better. I pushed the changes as requested.