run-docker should preserve an existing logdir/run_small file, if present

run-docker

@@ -1,6 +1,8 @@
 #!/bin/bash
 
 mkdir -p logdir
-cp run_small.in logdir/run_small
-chmod 755 logdir/run_small
+if [ ! -x logdir/run_small ]; then
+    cp run_small.in logdir/run_small
+    chmod 755 logdir/run_small
+fi
 docker run --rm --name walkingo_exp -v `/bin/pwd`/logdir:/home/walkingo/logdir walkingonions screen -D -m &