CI: Turn debian/12-x86_64 into a non-root container

Since commit 4611ae6fb8 ("CI: save toolstack artifact as cpio.gz"), the
various automation/scripts/* no longer need to be root to correctly repack the
initrd for test.

Signed-off-by: Javi Merino <javi.merino@cloud.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@vates.tech>

automation/build/debian/12-x86_64.dockerfile

@@ -9,6 +9,8 @@ RUN <<EOF
 #!/bin/bash
     set -eu
 
+    useradd --create-home user
+
     apt-get update
 
     DEPS=(
@@ -70,5 +72,5 @@ RUN <<EOF
     rm -rf /var/lib/apt/lists*
 EOF
 
-USER root
+USER user
 WORKDIR /build