Run as a non-root user

Running as a non-root user is a security best practice. Some environments require containers run as non-root users.

For the USER directive, a numeric uid is specified instead of the username because systems configured to disallow running images as root aren't able to run images that use user name string values for the USER because they can't validate that a named user isn't root. See https://github.com/kubernetes/kubernetes/pull/56503 for details.