Add docker tooling

Usage run "docker.sh " to execute in the docker image extracted from .gitlab-ci.yml in the emper root directory

Example: docker.sh make test

tools/docker-prepare

+#!/usr/bin/env bash
+set -euo pipefail
+
+useradd -u "${USER_ID}" -o -m user
+groupmod -g "${GROUP_ID}" user
+
+OUTSIDE_EMPER_ROOT="${1}"
+shift
+
+cd "${OUTSIDE_EMPER_ROOT}"
+
+# shellcheck disable=SC2068
+exec sudo -u user $@