feat(gpu): add ccache and layer splitting for faster builds

Port build speed improvements from PR OpenDroneMap#1959 to gpu.Dockerfile:
- Add ccache (CC/CXX/CCACHE_DIR env vars + apt install)
- Split COPY commands so system deps and SuperBuild compile layers
  are cached independently from application code changes
- Use RUN --mount=type=cache for the cmake/make step
- Copy docker/ early since PORTABLE_INSTALL needs the gcc/g++ wrappers

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: gartnera

gpu.Dockerfile

@@ -3,16 +3,36 @@ FROM nvidia/cuda:12.9.1-devel-ubuntu24.04 AS builder
 # Env variables
 ENV DEBIAN_FRONTEND=noninteractive \
     PYTHONPATH="$PYTHONPATH:/code/SuperBuild/install/local/lib/python3.12/dist-packages:/code/SuperBuild/install/lib/python3.12/dist-packages:/code/SuperBuild/install/bin/opensfm" \
-    LD_LIBRARY_PATH="$LD_LIBRARY_PATH:/code/SuperBuild/install/lib"
+    LD_LIBRARY_PATH="$LD_LIBRARY_PATH:/code/SuperBuild/install/lib" \
+    CC="ccache gcc" \
+    CXX="ccache g++" \
+    CCACHE_DIR=/ccache
 
 # Prepare directories
 WORKDIR /code
 
-# Copy everything
-COPY . ./
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends ccache && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/*
+
+# Copy build prerequisites first for better layer caching
+COPY snap/ ./snap/
+COPY configure.sh ./
+COPY requirements.txt ./
+COPY docker/ ./docker/
+
+# Install system dependencies
+RUN PORTABLE_INSTALL=YES GPU_INSTALL=YES bash configure.sh installreqs
 
-# Run the build
-RUN PORTABLE_INSTALL=YES GPU_INSTALL=YES bash configure.sh install
+COPY SuperBuild/ ./SuperBuild/
+
+# Compile SuperBuild with ccache
+RUN --mount=type=cache,target=/ccache \
+    PORTABLE_INSTALL=YES GPU_INSTALL=YES bash configure.sh install
+
+# Copy remaining source
+COPY . ./
 
 # Run the tests
 ENV PATH="/code/venv/bin:$PATH"