feat: add sklearnserver rockcraft.yaml

[DnPlas]

Adds sklearnserver rockcraft.yaml

Instructions to test

1. Build the rock

   rockcraft pack -v

2. Import the rock into a local docker registry

   # This step requires you to install skopeo, which you can do with apt on Ubuntu 20.04 or newer
   # https://github.com/containers/skopeo/blob/main/install.md
   sudo skopeo --insecure-policy copy oci-archive:<name of the rock file>.rock docker-daemon:<img name>:0.1

3. Run the container image

   docker run --rm <img name>:0.1

   Output