Work out the host and port for the container

Avoid docker inspect:
1. Use the container ID as the host
2. Configure the port, default to 3000

lib/kamal/configuration/docs/proxy.yml

@@ -56,6 +56,12 @@ proxy:
   # requests for other apps that do have a host set.
   host: foo.example.com
 
+  # Port
+  #
+  # The port the application is exposed on
+  # Defaults to 80
+  port: 3000
+
   # SSL
   #
   # Kamal Proxy can automatically obtain and renew TLS certificates for your applications.

lib/kamal/configuration/proxy.rb

@@ -25,6 +25,10 @@ class Kamal::Configuration::Proxy
     end
   end
 
+  def port
+    proxy_config.fetch("port", 80)
+  end
+
   def image
     proxy_config.fetch("image", DEFAULT_IMAGE)
   end