Add modular system package installation support

Implemented SYSTEM_PACKAGES environment variable for flexible system library installation:

Key Features:
- Users specify packages in .env: SYSTEM_PACKAGES="libopenslide0 libgdal-dev"
- Packages installed during Docker build (secure, no runtime mounting)
- Automatic rebuild detection when packages change
- Documentation with common scientific computing packages

Technical Implementation:
- ARG SYSTEM_PACKAGES in Dockerfile with conditional apt-get install
- Build argument passed from claude-docker.sh script
- Clean package management with cache cleanup

Benefits:
- Secure: No system directory mounting required
- Flexible: Users choose exactly what they need
- Documented: Common packages listed in README
- Clean: Packages baked into image, not mounted at runtime

Tested successfully with OpenSlide: conda run -n prism python -c "import openslide"

Dockerfile

@@ -12,6 +12,17 @@ RUN apt-get update && apt-get install -y \
     sudo \
     && rm -rf /var/lib/apt/lists/*
 
+# Install additional system packages if specified
+ARG SYSTEM_PACKAGES=""
+RUN if [ -n "$SYSTEM_PACKAGES" ]; then \
+    echo "Installing additional system packages: $SYSTEM_PACKAGES" && \
+    apt-get update && \
+    apt-get install -y $SYSTEM_PACKAGES && \
+    rm -rf /var/lib/apt/lists/*; \
+else \
+    echo "No additional system packages specified"; \
+fi
+
 # Create a non-root user with matching host UID/GID
 ARG USER_UID=1000
 ARG USER_GID=1000