build(ci): distribute build across multiple runners

[crazy-max]

Proposed changes

Adds an example using a matrix strategy to distribute the build for each platform across multiple runners and merge the resulting manifests into a single image using the buildx imagetools create command.

Related issues (optional)

fixes #16593

cc @kaovilai

[netlify]

✅ Deploy Preview for docsdocker ready!

Name	Link
🔨 Latest commit	b952032
🔍 Latest deploy log	https://app.netlify.com/sites/docsdocker/deploys/644798fc6fac5400081d5739
😎 Deploy Preview	https://deploy-preview-17180--docsdocker.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[kaovilai]

Looks awesome 😎

[dvdksn]

Using a local registry for this is so cool

[crazy-max]

I go ahead and merge this one. In follow-up I think we could provide a composite action for a better integration so user would not need to add extra steps to load, push the local image but also handling of imagetools command.

[jedevc]

SGTM 🎉

Realized I hadn't shared my thoughts from a DM with @crazy-max:

alternatively. we can use the push-by-digest key i think (might wanna check my sources on that), to only push the digest - then we grab the digests from the buildx output digest, and combine those