container: Rework uid/gid alignment with caller

Already create the builder user/group during container image build and
only align the IDs in the entrypoint if started with a non-zero USER_ID.
The primary gain is code simplification because this removes some
dynamics from the entrypoint.

As this refactoring avoids that gitlab-ci runners start the container as
root, it was also supposed to resolve the mismatch between the owner of
the checked-out repo and builder user. Unfortunately, this does not work
yet, and the reason is still unclear.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas-container

@@ -176,7 +176,7 @@ if [ -z "${KAS_CONTAINER_ENGINE}" ]; then
 	fi
 fi
 
-KAS_RUNTIME_ARGS="--log-driver=none"
+KAS_RUNTIME_ARGS="--log-driver=none --user=root"
 
 case "${KAS_CONTAINER_ENGINE}" in
 docker)