Add docker-setup hook

This allows the user to make any necessary configuration changes to
Docker before setting up any containers, allowing those configuration
changes to take effect from the outset.

lib/kamal/cli/templates/sample_hooks/docker-setup.sample

@@ -0,0 +1,7 @@
+#!/usr/bin/env ruby
+
+# A sample docker-setup hook
+#
+# Sets up a Docker network which can then be used by the application’s containers
+
+ssh user@example.com docker network create kamal