Don't self_build synapse Docker image on ARM

[mattcen]

As of Synapse 1.22, the Docker image is build not just for amd64, but also for armv7 and arm64. We should update the code to make use of these pre-built images rather than self-build on ARM.

I know that this line will need to change to include the new architectures:

https://github.com/spantaleev/matrix-docker-ansible-deploy/blob/master/group_vars/matrix_servers#L924

I'm not sure yet if any other changes are needed; doing some other stuff currently, so don't have a chance to dive into it in any greater detail yet.

[mattcen]

Ah, I just found 7d211b7, which discusses this.

I can't them on Docker Hub yet

There's a missing word here. Can't see them on Docker Hub? They're definitely there as of this comment.

[spantaleev]

We had a discussion in the Synapse Admins room about this (here). It seems like the amd64 image is being prioritized and pushed first. Images for other architectures follow about an hour later.

In 56ebeb1, I've reverted 7d211b7 just now, which fixes this issue.

Because non-amd64 images will always be pushed with a delay, we'll either have to delay playbook updates until all images are available (1-2 hours later), or we'll have to think of some other mechanism to handle ARM (perhaps a separate version variable for ARM, which we update independently?).

[mattcen]

Ah yes, I did notice that was happening when I was reviewing matrix-org/synapse#7921. I guess it didn't occur to me you'd be updating the playbook quite so rapidly after a new image release 😄

Yeah, I'm not sure of the best way to solve this one either.