rtd container uses wait-for-it, ensuring postfix before starting supe…

[rboyd]

…rvisord. fixes #5.

[floross]

I cannot test this pull request right now (in a few days probably). Is someone can test it before i merged it ? Or we will wait until I can.

[Jmennius]

@rboyd, thanks for the pull!
I'll review and test in a week or two (on vacation).

[Jmennius]

I have several points to consider:

1. Database is not the only service to wait for - we should also wait for ES and Redis.
2. Having the wait-for in compose, always enabled for DB. We should wait for DB conditionally (RTD_HAS_DATABASE) and do this in rtd-provision.sh script. That way we will cover every use-case.
3. wait-for-it.sh - instead of copying it to the repo (w/o any mention on where it comes from, revision, etc), we should download it in Dockerfile. That will significantly ease maintenance.

Thanks!