OMG…Instead of the env variable DOCKER_HOST: tcp://dockerproxy:2375 I had hostname: tcp://dockerproxy:2375 in there which only changed the website title. So dumb. Everything else was the same as your example.

Thanks for the help!

The macvlan routing solved it! I ended up using this page: https://blog.prabir.me/posts/creating-macvlan-in-synology-nas/

Thanks!