Use UID number rather than username in Dockerfile

[wdullaer]

Summary

Set a numeric user in the docker file, rather than the argocd username.

Motivation

Kubernetes will reject the pod if the security context does not allow to run as root, because it can't validate that a named user isn't root.
See: kubernetes/kubernetes#56503

I know you can overwrite the uid with a securityContext directive, but it'd be nice if this worked out of the box.

Proposal

Change https://github.com/argoproj/argo-cd/blob/master/Dockerfile#L78
to

USER 999

[jannfis]

We already have a PR for this: #3108

[jannfis]

And there is #2746

[wdullaer]

Nice to see it's already on the radar.
I did do a search through the issues, but apparently not thorough enough.

I'll be following the PR :-)