It’s not too complicated, you just have to run a container like so:
docker run -it --privileged --net=host <whatever Debian based container image you want to use> bash
This should give you a terminal inside this Debian based container. From there you just treat it like normal Debian:
# Inside the container
$ apt update
$ apt install ethtool
Then that’s it you should have ethtool installed in this container and should be able to use it inside this container. Due to --net=host and --privileged, you should be able to access the host ethernet interface from inside the container.
Best Regards,
Jeremias