Docker: explicitely set uid/gid to 999 for unit user.

This allows us to be consistent through possible updates of default
settings used in distributions.  Previous behaviour was uid/gid were
chosen automatically based on what uids/gids are already taken on the
system.

pkg/docker/template.Dockerfile

@@ -58,9 +58,9 @@ RUN set -ex \
     && @@RUN@@ \
     && mkdir -p /var/lib/unit/ \
     && mkdir /docker-entrypoint.d/ \
-    && groupadd --system unit \
+    && groupadd --gid 999 unit \
     && useradd \
-         --system \
+         --uid 999 \
          --gid unit \
          --no-create-home \
          --home /nonexistent \