add num processes to make build

Dockerfile

@@ -39,7 +39,7 @@ RUN git clone https://github.com/isl-org/Open3D.git && \
           -DUSE_SYSTEM_GLEW=OFF \
           -DUSE_SYSTEM_GLFW=OFF \
           .. && \
-    make && \
+    make -j$(nproc) && \
     make install-pip-package
 
 # copy files