ci(docker): build single architecture instead of multi arch image manifest

[bernardobridge]

Description

Using the docker/build-push-action uses buildx and sets an image manifest type which is unsupported by Lambda. We now build the image with docker build as before, but setup qemu to ensure emulation is present for building arm64.

Note: builds arm64 conditionally so we can skip in PR's for now (given how long it takes). We can then add smoke tests to main against canary.

Testing

Tested this in another branch with a workflow_dispatch build, and locally triggered some Lambda tests against arm64 and x86_64. All seems to be working.

Pre-merge checklist

• Does this require an update to the docs?
• Does this require a changelog entry?