dockerlib: short form for non-unique uid/gid for CentOS 5 compat (issue4977)

CentOS 5 does not support '--non-unique', but does support the short '-o'.

contrib/dockerlib.sh

@@ -35,8 +35,8 b' function initcontainer() {'
         # running docker. This is *very likely* to fail at some point.
         echo RUN useradd $DBUILDUSER -u 1000
     else
-        echo RUN groupadd $DBUILDUSER -g `id -g` --non-unique
-        echo RUN useradd $DBUILDUSER -u `id -u` -g $DBUILDUSER --non-unique
+        echo RUN groupadd $DBUILDUSER -g `id -g` -o
+        echo RUN useradd $DBUILDUSER -u `id -u` -g $DBUILDUSER -o
     fi
   ) | $DOCKER build --tag $CONTAINER -
 }