Use numeric user id to pass runAsNonRoot on Kubernetes

[sathieu]

Without this, we have:

container has runAsNonRoot and image has non-numeric user (memcache), cannot verify user is non-root

[LaurentGoderre]

I think this would be best address in your k8s deployment rather than in the image itself

[sathieu]

@LaurentGoderre Yes, we can do this, and hope the UID is not changed (it probably won't).

Merging this PR will reduce friction when using this image.

[tianon]

Duplicate of #79