containerize trac

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Trac should run inside a Docker container. I guess it wants to send emails sometimes - can trac itself be configured to connect directly to mailgun? Worst case we configure it like #266 describes, and just don't bother to expose any listening ports from the container, let exim deliver to mailgun via the same secret.

But honestly I feel like the emails could just break, most stuff happens on github these days.

Also, this can use a straight upstream postgres container, moving /var/lib/postgresql/9.1/main into a volume

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It also needs to talk to the commit-bot, see #269