So I was able to get a UCMA application to start/register and even accept a call running on a server core container, but it took some doing. The runtime needs to be installed in the container image, which needed to be done manually. You also need to provision the app using the container host's details, and get the cert into the container.
The place where it all breaks down though is with media-if your app needs to play/record prompts for calls, you can't do it from server core. Container or not, the OS is missing the runtime for WMA, and from what I've found, it can't be installed.
For what it's worth, the SFB team doesn't even support UCMA on server core, let alone core on a container, so you're dealing with a couple of layers of "not supported" stuff.