feat: remove default 'ubuntu' user on Ubuntu 23.04+

Starting from Ubuntu 23.04, the system includes a default 'ubuntu' user (UID 1000) to serve as a non-root OCI user. This cause errors when building the base image: `useradd: UID 1000 is not unique`.

Since we don't need the default 'ubuntu' user, this commit removes it.

Original PR: <kubeflow/kubeflow#7728>

Signed-off-by: Junlin Zhou <jameszhou2108@hotmail.com>

Author: edwardzjl

components/example-notebook-servers/base/Dockerfile

@@ -102,7 +102,10 @@ RUN curl -fsSL "https://dl.k8s.io/release/${KUBECTL_VERSION}/bin/linux/${TARGETA
  && chmod +x /usr/local/bin/kubectl
 
 # create user and set required ownership
-RUN useradd -M -N \
+# Starting with Ubuntu 23.04, the system includes a default user named 'ubuntu' with UID 1000,
+# which conflicts with our default NB_UID. As this user is not needed, it is removed.
+RUN userdel -r ubuntu || true \
+ && useradd -M -N \
     --shell /bin/bash \
     --home ${HOME} \
     --uid ${NB_UID} \