Use nproc to determine available CPUs for perallel execution

docker/build.sh

@@ -14,7 +14,7 @@ cp /usr/src/site.conf gluon/site/
 # Build
 cd gluon
 make update
-time make -j 5 GLUON_TARGET=ar71xx-generic
+time make -j $(($(nproc)+1)) GLUON_TARGET=ar71xx-generic
 
 set +x
 echo -e "\nBUILD FINISHED\n"