Consolidate container env settings

[cpeel]

Now that we can set sregistry config values with env variables (or will once https://github.com/singularityhub/sregistry/pull/409 merges), make it such that we only need to set them in one place in docker-compose for them to be picked up by all of the sregistry containers.

I also grouped the base service dependencies together at the top of the file.