ci: Do no build as root inside docker

I'm not sure exactly what are the security implications of running untrusted root processes inside docker, quick googling seems to say it means you're basically root on the host...

Even outside of security considerations, better build as non privileged user, to ensure we don't perform tasks that requires privilege. I see in glib Dockerfile it does this:

ARG HOST_USER_ID=5555
ENV HOST_USER_ID ${HOST_USER_ID}
RUN useradd -u $HOST_USER_ID -ms /bin/bash user

USER user
WORKDIR /home/user