ci: provide arm64 image build

[aldy505]

Effort for getsentry/self-hosted#1585

The GCB pipeline that build the image and push it to GCR will still be run, but for DockerHub, we'll take the image from GHCR (GitHub Container Registry) instead. This PR will provide ARM64 version available on GHCR and DockerHub.

Also see the comment on snuba PR, we were talking about building it once and mirroring that image everywhere (build on GitHub Actions, push to GitHub Container Registry, DockerHub and GCR): getsentry/snuba#6800 (comment)

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

[bc-sentry]

@aldy505 While arm64 runners are in beta, the global pool of runners is severely constrained and will result in job queuing. We would like to wait to add the arm64 image until runners is prod and sufficient capacity exists. Thanks.