Use docker managed volumes in docker-compose

[adarshpk]

This is so that the compose file works across different platforms with no issues (I had an issue regarding permission to /opt directory).

[timabbott]

@adarshpk thanks for the pull request! I guess https://github.com/zulip/docker-zulip/pull/166/files is actually the same; we should definitely merge one of these.

Do you know what the conventions are in the Docker community on naming volumes?

[adarshpk]

@timabbott I'm unaware of the volume naming conventions if any.

[jasonhildebrand]

I have used many docker-compose based projects over the past several years. There are no hard conventions. The proposed names in the PR are sensible.

I would support merging this PR to improve zulip's alignment with docker best practices.

Note that docker-compose will "namespace" the volumes by prepending the project name (which defaults to the directory containing the docker-compose.yml file). This means the actual docker volumes will typically be named zulip_postgresq, zulip_rabbitmq, etc.

[i-ky]

#166 is now closed by the author. @timabbott, please consider merging this PR. You raised backward compatibility concerns in https://github.com/zulip/docker-zulip/pull/166#issuecomment-437986089, but I think that https://github.com/zulip/docker-zulip/pull/166#issuecomment-615525202 answers them pretty well.