So when in the traefik container you reference 127.0.0.1 you are not referencing the docker host, your are referencing the traefik container itself. You need to use one of the addressable ips of the host.
Personally I would move collectd into a container as it make it easily addressable by host and other containers.
If you are running docker for mac/windows you can utilise the special hostname host.docker.internal to address the container host. I don't like using this, as it does not translate well into linux.