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

lib/kamal/commands/proxy.rb

@@ -2,14 +2,14 @@ class Kamal::Commands::Proxy < Kamal::Commands::Base
   delegate :argumentize, :optionize, to: Kamal::Utils
 
   def run
-    docker :run,
-      "--name", container_name,
-      "--network", "kamal",
-      "--detach",
-      "--restart", "unless-stopped",
-      "--volume", "kamal-proxy-config:/home/kamal-proxy/.config/kamal-proxy",
-      "\$\(#{get_boot_options.join(" ")}\)",
-      config.proxy_image
+    pipe \
+      [ :echo, "\$\(#{get_boot_options.join(" ")}\) #{config.proxy_image}" ],
+      xargs(docker(:run,
+        "--name", container_name,
+        "--network", "kamal",
+        "--detach",
+        "--restart", "unless-stopped",
+        "--volume", "kamal-proxy-config:/home/kamal-proxy/.config/kamal-proxy"))
   end
 
   def start
@@ -73,7 +73,7 @@ class Kamal::Commands::Proxy < Kamal::Commands::Base
   end
 
   def get_boot_options
-    combine [ :cat, config.proxy_options_file ], [ :echo, "\"#{config.proxy_options_default.join(" ")}\"" ], by: "||"
+    combine [ :cat, config.proxy_options_file, "2>", "/dev/null" ], [ :echo, "\"#{config.proxy_options_default.join(" ")}\"" ], by: "||"
   end
 
   def reset_boot_options