extras/dialer: allow to seed random

Helpful if you want to get the same random numbers in subsequent runs
(e.g. to compare results between branches).

extras/dialer/dialer

@@ -1,13 +1,17 @@
 #!/bin/bash
 set -eu
 if [ $# -lt 1 ]; then
-    echo "Usage: $0 <ip:port> [<max_dialer>]" >&2
+    echo "Usage: $0 <ip:port> [<max_dialer> [<random_seed>]]" >&2
     exit 1
 fi
 
 readonly addr=$1
 readonly max_dialer=${2:-50}
 
+if [[ $# -gt 2 ]]; then
+    RANDOM=$3
+fi
+
 dialer=()
 # shellcheck disable=SC2154
 trap 'echo -n "stopping ... "; for c in "${dialer[@]}"; do docker rm -f "$c" >/dev/null; done; echo "done"' EXIT