use dirclean instead of clean between builds

docker/build.sh

@@ -17,15 +17,14 @@ cd gluon
 make update
 
 time make -j $(($(nproc)+1)) V=s BROKEN=1 GLUON_TARGET=ar71xx-generic
-make clean
 time make -j $(($(nproc)+1)) V=s BROKEN=1 GLUON_TARGET=ar71xx-nand
-make clean
+make dirclean
 time make -j $(($(nproc)+1)) V=s BROKEN=1 GLUON_TARGET=mpc85xx-generic
-make clean
+make dirclean
 time make -j $(($(nproc)+1)) V=s BROKEN=1 GLUON_TARGET=x86-generic
-make clean
+make dirclean
 time make -j $(($(nproc)+1)) V=s BROKEN=1 GLUON_TARGET=x86-64
-make clean
+make dirclean
 
 set +x
 echo -e "\nBUILD FINISHED\n"