feat: add Red Hat UBI9 minimal base image

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Summary

Adds a minimal Red Hat UBI9 base image for Coder workspaces, providing enterprise-grade security and compliance while maintaining a minimal footprint.

Features

Base Operating System

• Red Hat UBI9: Enterprise-grade, security-focused base image
• Enterprise Ready: Red Hat supported with regular security updates
• Compliance: Meets enterprise security and compliance requirements
• Minimal: Only essential packages included

Included Tools

• Development Tools: GCC, make, and essential build tools
• Docker CE: Container development and deployment
• Git: Version control
• Python 3: System Python with pip
• System Utilities: curl, wget, jq, htop, vim, unzip, rsync, systemd

Design Approach

Follows the same minimal pattern as the existing Ubuntu base image:

• Single-stage build for simplicity
• Essential packages only
• Designed to be extended by users
• Non-root coder user with Docker group access

Use Cases

This base image is ideal for:

• Enterprise environments requiring Red Hat compliance
• Container development with Docker
• Custom development images (extend with specific language runtimes)
• Security-conscious deployments with minimal attack surface

Files Added

• images/redhat-ubi9/ubi9.Dockerfile - Minimal Dockerfile following Ubuntu base pattern
• images/redhat-ubi9/README.md - Comprehensive documentation with extension examples

Testing

• ✅ Dockerfile syntax validated
• ✅ Package installations verified
• ✅ User permissions configured correctly
• ✅ Follows coder/images repository conventions

This provides Coder users with an enterprise-ready, minimal base image that can be extended for specific development needs while maintaining Red Hat compliance.