Redirect buildx build output to stdout

Docker buildx build outputs the build logs to stderr by default.
SSHKit displays stderr logs in red, which can suggest that an error has
occurred.

Redirect the output to stdout, so it shows in green. If there is an
error, the output will be repeated in red anyway.

Fixes: https://github.com/basecamp/kamal/issues/1356

lib/kamal/commands/builder/base.rb

@@ -20,7 +20,8 @@ class Kamal::Commands::Builder::Base < Kamal::Commands::Base
       *([ "--builder", builder_name ] unless docker_driver?),
       *build_tag_options(tag_as_dirty: tag_as_dirty),
       *build_options,
-      build_context
+      build_context,
+      "2>&1"
   end
 
   def pull