support/dockerfile: add directives to run as non-root

support/dockerfile: add directives to run as non-root

Currently, our jobs on the gitlab-ci infra are running as root, which is
problematic for two reasons:

  - this is not the usual way Buildroot is built;
  - it may miss issues where running as non-root is problematic.

So, complement our Dockerfile with directives to add a new user and run
everything as that user, as demonstrated by this build job:
    https://gitlab.com/ymorin/buildroot-ci/-/jobs/46929562

Additional, enforce an UTF-8 locale while running.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>