tests/docker: force NOUSER=1 for base images

tests/docker: force NOUSER=1 for base images

As base images are often used to build further images like toolchains
ensure we don't add the local user by accident. The local user should
only exist on local images and not anything that gets pushed up to the
public registry.

Reported-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20211115142915.3797652-2-alex.bennee@linaro.org>