Publish unversioned Docker image tags from main branch

[bdice]

Summary

Closes #645.

This PR splits the image tag into two components:

• version: The RAPIDS version (e.g., 26.02)
• base_tag: The image variant identifier (e.g., cpp-cuda13.1-ubuntu2404)

The full versioned tag is constructed as ${version}-${base_tag}.

Changes

1. .github/actions/devcontainer-json/action.sh: Output base_tag and version separately instead of a combined tag.

2. .github/actions/devcontainer-json/action.yml: Expose both base_tag and version outputs.

3. .github/workflows/build-test-and-push-linux-image.yml:

   • Build images with tag ${version}-${base_tag}
   • Push versioned manifests (always)
   • Push unversioned manifests using ${base_tag} (main branch only)

Example Tags

Branch	Tags
main	26.02-cpp-cuda13.1-ubuntu2404, 26.02-cpp-cuda13.1, cpp-cuda13.1-ubuntu2404, cpp-cuda13.1
release/26.02	26.02-cpp-cuda13.1-ubuntu2404, 26.02-cpp-cuda13.1

[gforsyth]

There's one wonky comment, but otherwise I think this is good to go.