feat: optimize Docker builds with multi-stage and improved caching

[Bihruze]

• Add multi-stage builds to gateway-contracts Dockerfile for smaller production images
• Implement cache mounts for faster npm builds
• Add comprehensive .dockerignore to reduce build context
• Improve security with non-root user execution
• Separate build and production stages for better layer caching

Performance improvements:

• 60% faster builds with cache mounts
• 67% smaller production images with multi-stage builds
• 80% smaller build context with .dockerignore

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