Use xargs to handle spaces in proxy options

We cat the options file, append the proxy image and then pass it
to xargs to ensure it handles spaces correctly.

Works better than using eval which can handle spaces but tries
to evaluate things like backticks.

Fixes: https://github.com/basecamp/kamal/issues/1448

test/integration/docker/deployer/app_with_traefik/.kamal/hooks/pre-deploy

@@ -1,3 +1,4 @@
 kamal proxy boot_config set --publish false \
                             --docker_options label=traefik.http.services.kamal_proxy.loadbalancer.server.scheme=http \
-                                             label=traefik.http.routers.kamal_proxy.rule=PathPrefix\(\`/\`\)
+                                             label=traefik.http.routers.kamal_proxy.rule=PathPrefix\(\`/\`\) \
+                                             sysctl=net.ipv4.ip_local_port_range=\"10000\ 60999\"

test/integration/proxy_test.rb

@@ -46,7 +46,27 @@ class ProxyTest < IntegrationTest
     logs = kamal :proxy, :logs, capture: true
     assert_match /No previous state to restore/, logs
 
+    kamal :proxy, :boot_config, :set, "--docker-options='sysctl net.ipv4.ip_local_port_range=\"10000 60999\"'"
+    assert_docker_options_in_file
+
+    kamal :proxy, :reboot, "-y"
+    assert_docker_options_in_container
+
+    kamal :proxy, :boot_config, :reset
+
     kamal :proxy, :remove
     assert_proxy_not_running
   end
+
+  private
+    def assert_docker_options_in_file
+      boot_config = kamal :proxy, :boot_config, :get, capture: true
+      assert_match "Host vm1: --publish 80:80 --publish 443:443 --log-opt max-size=10m --sysctl net.ipv4.ip_local_port_range=\"10000 60999\"", boot_config
+    end
+
+    def assert_docker_options_in_container
+      assert_equal \
+        "{\"net.ipv4.ip_local_port_range\":\"10000 60999\"}",
+        docker_compose("exec vm1 docker inspect --format '{{ json .HostConfig.Sysctls }}' kamal-proxy", capture: true).strip
+    end
 end