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 @@ def bootstrap
     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

test/cli/server_test.rb

@@ -23,10 +23,14 @@ class CliServerTest < CliTestCase
     SSHKit::Backend::Abstract.any_instance.expects(:execute).with('[ "${EUID:-$(id -u)}" -eq 0 ] || command -v sudo >/dev/null || command -v su >/dev/null', raise_on_non_zero_exit: false).returns(true).at_least_once
     SSHKit::Backend::Abstract.any_instance.expects(:execute).with(:curl, "-fsSL", "https://get.docker.com", "|", :sh).at_least_once
     SSHKit::Backend::Abstract.any_instance.expects(:execute).with(:mkdir, "-p", ".kamal").returns("").at_least_once
+    Kamal::Commands::Hook.any_instance.stubs(:hook_exists?).returns(true)
+    hook_variables = { version: 999, service_version: "app@999", hosts: "1.1.1.1,1.1.1.2,1.1.1.3,1.1.1.4", command: "server", subcommand: "bootstrap" }
+    SSHKit::Backend::Abstract.any_instance.expects(:execute).with(".kamal/hooks/docker-setup", anything).at_least_once
 
     run_command("bootstrap").tap do |output|
       ("1.1.1.1".."1.1.1.4").map do |host|
         assert_match "Missing Docker on #{host}. Installing…", output
+        assert_match "Running the docker-setup hook", output
       end
     end
   end

test/integration/docker/deployer/app/.kamal/hooks/docker-setup

@@ -0,0 +1,3 @@
+#!/bin/sh
+echo "Docker set up!"
+mkdir -p /tmp/${TEST_ID} && touch /tmp/${TEST_ID}/docker-setup