feat: Add manual Docker push workflow

[rossnelson]

Summary

• Add workflow_dispatch action to manually build and push Docker images
• Allows specifying any git ref (commit, tag, branch) to build from
• Enables custom version tagging without creating new releases

Purpose

This workflow provides a way to fix incorrectly tagged Docker images without the side effects of creating a new GitHub release (which triggers downstream systems, notifications, and version bumps).

Usage

The workflow can be triggered manually from the Actions tab with:

• ref: Git reference to build from (e.g., v2.38.3, commit SHA, branch name)
• version: Docker tag to apply (e.g., 2.38.3)
• push_latest: Whether to also update the latest tag
• image_type: Which images to push (ui, cloud-ui, or both)

Test plan

• Workflow file is valid YAML
• Uses existing Docker setup and build-push actions
• Conditional logic correctly handles image type selection