ci(release.yml): Build and push images for amd64 and arm64/v8

[erikmd]

• Kind: enhancement

• Close Task: port multi-platform support from deploy-master.yml to release.yml #626

Description

Just amounts to porting the following patches:

• af5b77a
• 7ba32cd
• 3431025

This PR will be ultimately tested at the upcoming point release, but given it's difficult to re-test it in advance,
I will keep this one opened for a few days, in case other devs want to take a look and maybe suggest enhancements.

Checklist

• Read the CONTRIBUTING.md guide and:
  • Use Atomic Commits so each commit gathers a single logical change
  • Use Conventional Commits regarding commit messages (needed by our release toolchain)

Note to maintainers

• Read this wiki page.
• Make sure the PR has a milestone.
• Assign yourself before merging.
• Either do a regular merge:
  • for PRs containing several commits following conventional-commits,
  • or for PRs containing 1 commit shared with a later PR (to preserve the SHA1)
• Or do a squash-merge:
  • for PRs containing only 1 commit (not shared with a later PR),
  • or for PRs containing several commits that need not be kept in the history;
  • → Update the commit message header with a conventional-commit type,
  • → Add a footer Close #… if a related issue exists.