dockerlib: allow non-unique uid and gid of $DBUILDUSER (issue4657)

There are make targets for building mercurial packages for various
distributions using docker. One of the preparation steps before building is to
create inside the docker image a user with the same uid/gid as the current user
on the host system, so that the resulting files have appropriate
ownership/permissions.

It's possible to run `make docker-<distro>` as a user with uid or gid that is
already present in a vanilla docker container of that distibution. For example,
issue4657 is about failing to build fedora packages as a user with uid=999 and
gid=999 because these ids are already used in fedora, and groupadd fails.
useradd would fail too, if the flow ever got to it (and there was a user with
such uid already).

A straightforward (maybe too much) way to fix this is to allow non-unique uid
and gid for the new user and group that get created inside the image. I'm not
sure of the implications of this, but marmoute encouraged me to try and send
this patch for stable.

contrib/dockerlib.sh

@@ -35,8 +35,8 b' function initcontainer() {'
         # running docker. This is *very likely* to fail at some point.
         echo RUN useradd $DBUILDUSER -u 1000
     else
-        echo RUN groupadd $DBUILDUSER -g `id -g`
-        echo RUN useradd $DBUILDUSER -u `id -u` -g $DBUILDUSER
+        echo RUN groupadd $DBUILDUSER -g `id -g` --non-unique
+        echo RUN useradd $DBUILDUSER -u `id -u` -g $DBUILDUSER --non-unique
     fi
   ) | $DOCKER build --tag $CONTAINER -
 }