Hey @4Bob, welcome to the forum!
To be honest, if I have a host, where I can install docker, personally I wouldn’t bother trying to run CMK in a container, I’d simply install the appropriate package for my distribution and be done with it, to not have to deal with any quirks coming from inside the container environment.
Be that as it may, I’m not sure why you want to run either “check_icmp” or anything as root for that matter. One is supposed to run everything with the siteuser, which in your case is apparently cmk. As it says in your site creation output: Switch to your siteuser with omd su and run whatever you want to run.
Perhaps this is not the answer you wanted to hear, but it may get you a step further, I hope.
Regards,
Thomas