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).