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/server.rb

@@ -19,5 +19,7 @@ class Kamal::Cli::Server < Kamal::Cli::Base
     if missing.any?
       raise "Docker is not installed on #{missing.join(", ")} and can't be automatically installed without having root access and the `curl` command available. Install Docker manually: https://docs.docker.com/engine/install/"
     end
+
+    run_hook "docker-setup"
   end
 end

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