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>

support/docker/Dockerfile

@@ -28,3 +28,10 @@ RUN apt-get -q -y clean
 RUN sed -i 's/# \(en_US.UTF-8\)/\1/' /etc/locale.gen
 RUN /usr/sbin/locale-gen
 
+RUN useradd -ms /bin/bash br-user
+RUN chown -R br-user:br-user /home/br-user
+
+USER br-user
+WORKDIR /home/br-user
+ENV HOME /home/br-user
+ENV LC_ALL en_US.UTF-8