* add synapse-auto-compressor workaround, fixes #3397

* Clarify what the PG-prefixed variables are for

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Co-authored-by: Slavi Pantaleev <slavi@devture.com>

This provides an additional security benefit. The password won't leak in
the process list anymore.

It will, additionally, be connected to the devture-postgres network, if
devture-postgres is enabled.