Use docker info to get arch

lib/kamal/configuration/builder.rb

@@ -29,9 +29,7 @@ class Kamal::Configuration::Builder
 
   def local_arches
     @local_arches ||= if remote
-      uname_m = `uname -m`.strip
-      local_arch = uname_m == "x86_64" ? "amd64" : uname_m
-      arches & [ local_arch ]
+      arches & [ Kamal::Utils.docker_arch ]
     else
       arches
     end

lib/kamal/utils.rb

@@ -81,4 +81,16 @@ module Kamal::Utils
   def join_commands(commands)
     commands.map(&:strip).join(" ")
   end
+
+  def docker_arch
+    arch = `docker info --format '{{.Architecture}}'`.strip
+    case arch
+    when /aarch64/
+      "arm64"
+    when /x86_64/
+      "amd64"
+    else
+      arch
+    end
+  end
 end