Do not allow both host and hosts for proxy configuration

lib/kamal/configuration/docs/proxy.yml

@@ -31,7 +31,7 @@ proxy:
   # Hosts
   #
   # The hosts that will be used to serve the app. The proxy will only route requests
-  # to this host to your app.
+  # to these hosts to your app.
   #
   # If no hosts are set, then all requests will be forwarded, except for matching
   # requests for other apps deployed on that server that do have a host set.

lib/kamal/configuration/validator/proxy.rb

@@ -6,6 +6,10 @@ class Kamal::Configuration::Validator::Proxy < Kamal::Configuration::Validator
       if config["host"].blank? && config["hosts"].blank? && config["ssl"]
         error "Must set a host to enable automatic SSL"
       end
+
+      if config["host"].present? && config["hosts"].present?
+        error "Must use either 'host' or 'hosts', not both"
+      end
     end
   end
 end

test/configuration/proxy_test.rb

@@ -28,6 +28,11 @@ class ConfigurationProxyTest < ActiveSupport::TestCase
     assert_raises(Kamal::ConfigurationError) { config.proxy.ssl? }
   end
 
+  test "ssl with both host and hosts" do
+    @deploy[:proxy] = { "ssl" => true, host: "example.com", hosts: [ "anotherexample.com" ] }
+    assert_raises(Kamal::ConfigurationError) { config.proxy.ssl? }
+  end
+
   test "ssl false" do
     @deploy[:proxy] = { "ssl" => false }
     assert_not config.proxy.ssl?