nix-unstable-static: add run_as_user.sh script

Allow to run nix as a user.

images/nix-unstable-static/run_as_user.sh

@@ -0,0 +1,29 @@
+#!/bin/sh -eu
+#
+# This script can be used as an entrypoint. It's used to synchronize and run
+# the container with the same user and group as the host user.
+#
+# Usage:
+#   user_id=$(id -u) user_name=$(id -un) \
+#   group_id=$(id -g) group_name=$(id -gn) \
+#   ./run_as_user.sh [<command> ...<args>]
+#
+# shellcheck disable=SC2154
+
+# Install the host user and group into the container
+delgroup "${group_name}" 2>/dev/null || true
+deluser "${user_name}" 2>/dev/null || true
+
+addgroup -g "${group_id}" "${group_name}"
+adduser -D -G "${group_name}" -u "${user_id}" "${user_name}"
+
+# Don't propagate those env vars
+user=${user_name}
+unset user_id user_name group_id group_name
+
+# Change into the user
+if [ $# = 0 ]; then
+  exec su "${user}"
+else
+  exec su "${user}" -c /bin/sh /bin/sh -c "exec \"\$@\"" "$@"
+fi