Update Docker instructions (#13437)

substrate/docker/run.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+
+args=$@
+
+# handle when arguments not provided. run arguments provided to script.
+if [ "$args" = "" ] ; then
+    printf "Note: Please try providing an argument to the script.\n\n"
+    exit 1
+else
+    printf "*** Running Substrate Docker container with provided arguments: $args\n\n"
+    docker run --rm -it parity/substrate $args
+fi
+