Short-term fix for https://github.com/matrix-org/synapse/issues/8766.

This bumps us closer to current Python without going all the way to 3.9.

Fixes #8674

Signed-off-by: Dan Callahan <danc@element.io>

Signed-off-by: Christopher May-Townsend <chris@maytownsend.co.uk>

As mentioned in #7397, switching to a debian base should help with multi-arch work to save time on compiling. This is unashamedly based on #6373, but without the extra functionality. Switch python version back to generic 3.7 to always pull the latest. Essentially, keeping this as small as possible. The image is bigger though unfortunately.

* Add libwebp dependency to Dockerfile

Signed-off-by: Juho Vanhanen <juho@vanhanen.io>

Alpine Linux 3.8 is still supported, but it seems like
it's quite outdated now.

While Python should be the same on both, all other libraries, etc.,
are much newer in Alpine 3.9 and 3.10.

Signed-off-by: Slavi Pantaleev <slavi@devture.com>

Signed-off-by: Amir Zarrinkafsh <nightah@me.com>

Python 3.7 is apparently faster than 3.6, and should be mature enough.

Fixes #5467.

Also:

* Fix wrapping in docker readme
* Clean up some docs on the docker image
* a workaround for #4554

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.

Rewrite the dockerfile as a multistage build: this means we can get rid of a whole load of cruft which we don't need.

This addresses #3224

This matches docker best practices.

Signed-off-by: Benedikt Heine <bebe@bebehei.de>

Signed-off-by: Paul Tötterman <paul.totterman@iki.fi>