There are two reasons this is a good thing:

 * first, it means that you don't end up with stuff kicking around your working
   copy ending up in the build image by mistake (which can upset the pip
   install process)

 * second: it means that the docker image cache is more effective, and we can
   reuse docker images when iterating on the docker stuff.

Fixes #4559

Switch the matrixdotorg/synapse:latest Docker image to use python 3

... basically, carry on and fall back to SRV etc.

I can't figure out what's going on with #4422 and #4436; perhaps this will help.

* update debian installation instructions

* docs PR is docs

* Fix replication for room v3

We were not correctly quoting the path fragments over http replication,
which meant that it exploded when the event IDs had a slash in them

* Newsfile

* Handle listening for ACME requests on IPv6 addresses

the weird url-but-not-actually-a-url-string doesn't handle IPv6 addresses
without extra quoting. Building a string which you are about to parse again
seems like a weird choice. Let's just use listenTCP, which is consistent with
what we do elsewhere.

* Clean up the default ACME config

make it look a bit more consistent with everything else, and tweak the defaults
to listen on port 80.

* newsfile

Signed-off-by: Robert Jacob <xperimental@solidproject.de>

I got fed up with always adding '@unittest.DEBUG' every time I needed to debug a test.

* fix obvious problem :|

* changelog