add docker tooling

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

NOTE: seccomp filtering is disabled for now since io_uring_* syscalls
are not working everywhere as expected.

docker.sh

+#!/usr/bin/env bash
+
+# The directory of this script is also EMPER's root directory.
+EMPER_ROOT="$( cd "$( dirname "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"
+
+IMAGE=$(sed --regexp-extended --quiet 's;^image: "([^"]*)"$;\1;p' "${EMPER_ROOT}/.gitlab-ci.yml")
+
+docker run \
+	   --volume="${EMPER_ROOT}:${EMPER_ROOT}" \
+	   --interactive \
+	   --tty \
+	   --env USER_ID="${UID}" \
+	   --env GROUP_ID="$(id -g ${USER})" \
+	   --security-opt=seccomp:unconfined \
+	   "${IMAGE}" \
+	   "${EMPER_ROOT}/tools/docker-prepare" "${EMPER_ROOT}" $@

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 $@