Add IPv6 support for Docker images

[LouisMT]

Currently, Synapse doesn't work using IPv6 (see #1002). A fix is described in #2088 and has been applied to the Dockerfile in this PR.

I understand that Debian doesn't have Twister 17 yet, but as this is a Docker image it doesn't really matter, so just install Twister 17 to make it work. The image size hasn't notably changed, as it's 492 MB before and after.

As Docker provides an IPv6 DNS server by default in custom networks (which are required to link containers since --link is deprecated) I think it's pretty important that IPv6 is supported in the images.

Thanks!

[matrixbot]

Can one of the admins verify this patch?

[matrixbot]

Can one of the admins verify this patch?

[richvdh]

I'm surprised this is necessary. why does the pip install command at https://github.com/matrix-org/synapse/pull/3551/files#diff-3254677a7917c6c01f55212f86c57fbfR11 not install the latest released Twisted?

(unless the base image already has an old version of Twisted, but that seems unlikely?)

[LouisMT]

You're right, I saw that version 18.4 is already installed while testing. Forgot to update the PR.

I'm getting a lot of 401s exactly as described in #1002, but I'll do some more research to find out what's causing it.

Thanks!

[LouisMT]

Turns out I didn't read the reverse proxy docs well enough:
https://github.com/matrix-org/synapse#using-a-reverse-proxy-with-synapse