Publish unversioned Docker image tags from main branch

[bdice]

Summary

We should publish Docker images without RAPIDS version prefixes when building from the main branch, enabling users to pull the latest development images without specifying a version.

Current Behavior

All Docker images are published with RAPIDS version prefixes:

• rapidsai/devcontainers:26.02-cpp-cuda13.1-ubuntu2404
• rapidsai/devcontainers:26.02-cpp-cuda13.1

Proposed Behavior

On pushes to the main branch, publish both versioned and unversioned tags:

• Versioned: rapidsai/devcontainers:26.02-cpp-cuda13.1-ubuntu2404
• Unversioned: rapidsai/devcontainers:cpp-cuda13.1-ubuntu2404

On all other branches, continue publishing only versioned tags.

Prior Art

This follows the pattern established in rapidsai/ci-imgs:

• double-tag images (with and without RAPIDS version) ci-imgs#298
• Only publish unversioned Docker tags from main branch ci-imgs#311