Publish multi-arch images for agentk in addition to the single-arch ones

Problem to solve

In !592 (merged) we added -arm64 suffixed images for agentk, leaving the pre-existing tags as AMD64 only. It would be better if the "clean" tags were multi-arch.

Proposal

Make the primary agentk image tag multi-arch using docker manifest commands as outlined in the "hard path" section of https://www.docker.com/blog/multi-arch-build-and-images-the-simple-way/

Background

The following discussion from !592 (merged) should be addressed:

• @hfyngvason started a discussion: (+3 comments)

  @ash2k So there's tag per architecture? Can we build multiarch images too? So a single tag works on all architectures? This is what we do with auto-deploy-image, GitLab Runner etc.