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.

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