Add a pre-deploy hook

Useful for checking the status of CI before deploying. Doing this at
this point in the deployment maximises the parallelisation of building
and running CI.

test/cli/build_test.rb

@@ -9,7 +9,11 @@ class CliBuildTest < CliTestCase
   end
 
   test "push" do
+    Mrsk::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: "build", subcommand: "push" }
+
     run_command("push").tap do |output|
+      assert_hook_ran "pre-build", output, **hook_variables
       assert_match /docker --version && docker buildx version/, output
       assert_match /docker buildx build --push --platform linux\/amd64,linux\/arm64 --builder mrsk-app-multiarch -t dhh\/app:999 -t dhh\/app:latest --label service="app" --file Dockerfile \. as .*@localhost/, output
     end