add multi stage build to netdisco-base

docker/netdisco-base/wait_to_start.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+
+echo $WAIT_COMMAND
+echo $WAIT_START_CMD
+
+is_ready() {
+    eval "$WAIT_COMMAND"
+}
+
+# wait until is ready
+i=0
+while ! is_ready; do
+    i=`expr $i + 1`
+    if [ $i -ge $WAIT_LOOPS ]; then
+        echo "$(date) - still not ready, giving up"
+        exit 1
+    fi
+    echo "$(date) - waiting to be ready"
+    sleep $WAIT_SLEEP
+done
+
+#start the script
+exec $WAIT_START_CMD