Use distroless image as the base image, run Katib components as a non-root user

/kind discussion

Once the ephemeral debug containers feature is available by default, we might better change the base image from the alpine to the distroless image, run Katib components as a non-root user.

Ref:
- https://github.com/kubeflow/katib/pull/1662#discussion_r706635579
- [Debugging with an ephemeral debug container
](https://kubernetes.io/docs/tasks/debug-application-cluster/debug-running-pod/#ephemeral-container)

Review the discussion in PR #1662 and the linked Kubernetes documentation on ephemeral debug containers first. Then identify the Katib component images and runtime user settings; completion requires distroless bases and non-root execution once ephemeral debug containers are available.

Written by the indexing model from the issue text.