Optimize Dockerfile for Fast Rebuilds

[abhibongale]

Analyze rebuild times from Issue #14 test runs and further optimize the Ironic Dockerfile to achieve sub-2-minute rebuilds. Focus on layer caching, BuildKit features, and minimizing what changes trigger full rebuilds.

The Dockerfile from Issue #4 is already optimized, but this issue measures actual performance and makes data-driven improvements.

Goals

• Measure current rebuild times
• Identify rebuild bottlenecks
• Optimize layer caching strategy
• Use BuildKit cache mounts
• Achieve consistent < 2 minute rebuilds
• Document optimization techniques

Tasks

• Measure baseline rebuild times
• Analyze Docker layer cache hits/misses
• Add BuildKit cache mounts for pip
• Optimize COPY ordering
• Test with real Ironic code changes
• Measure optimized rebuild times
• Document optimization gains
• Update Dockerfile with improvements

Acceptance Criteria

• Full build < 5 minutes
• Code-only rebuild < 2 minutes
• Requirements-only rebuild < 3 minutes
• Cache hit rate > 80% for code changes
• Build times are consistent
• Optimization documented