Eval proxy options

To ensure that the proxy options are evaluated as if they were
added verbatim to the command, we'll eval the docker run command.

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

lib/kamal/commands/base.rb

@@ -76,6 +76,10 @@ module Kamal::Commands
         [ :sh, "-c", "'#{command.flatten.join(" ").gsub("'", "'\\\\''")}'" ]
       end
 
+      def eval(*args)
+        [ :eval, *args ]
+      end
+
       def docker(*args)
         args.compact.unshift :docker
       end

lib/kamal/commands/proxy.rb

@@ -2,14 +2,21 @@ 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
+    shell \
+      chain \
+        boot_options,
+        eval(
+          docker(
+            :run,
+            "--name", container_name,
+            "--network", "kamal",
+            "--detach",
+            "--restart", "unless-stopped",
+            "--volume", "kamal-proxy-config:/home/kamal-proxy/.config/kamal-proxy",
+            "\$OPTIONS",
+            config.proxy_image
+          )
+        )
   end
 
   def start
@@ -72,6 +79,10 @@ class Kamal::Commands::Proxy < Kamal::Commands::Base
     remove_directory config.proxy_directory
   end
 
+  def boot_options
+    "OPTIONS=$(cat #{config.proxy_options_file} 2> /dev/null || echo \"#{config.proxy_options_default.join(" ")}\")"
+  end
+
   def get_boot_options
     combine [ :cat, config.proxy_options_file ], [ :echo, "\"#{config.proxy_options_default.join(" ")}\"" ], by: "||"
   end