chore: optimize debian-dev Dockerfiles for better layer caching

[sihyeonn]

Summary

• Optimize debian-dev/Dockerfile and debian-dev/Dockerfile.local for better Docker layer caching
• Copy only dependency-related files before full source code to improve cache hits on code changes

Changes

Layer Caching Optimization

• Dockerfile.local: Separate COPY steps - first copy Makefile, rockspec, .requirements, then run make deps, finally copy remaining source code
• This ensures make deps layer is cached when only application code changes

Brotli Build Improvement

• Move brotli compilation into Dockerfile as separate RUN layer (cached independently)
• Copy pre-built brotli from build stage instead of rebuilding in production stage

Cleanup

• Remove duplicate apt-get install libldap2-dev in production stage
• Fix ENV syntax (DEBIAN_FRONTEND=noninteractive instead of DEBIAN_FRONTEND noninteractive)

Benefits

• Faster rebuilds: Code-only changes no longer trigger full dependency reinstallation
• Smaller final image: Brotli build tools not installed in production stage
• Simpler maintenance: No external shell script dependency