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

@@ -1,42 +1,42
 #!/bin/sh -eu
 
 # This function exists to set up the DOCKER variable and verify that
 # it's the binary we expect. It also verifies that the docker service
 # is running on the system and we can talk to it.
 function checkdocker() {
   if which docker.io >> /dev/null 2>&1 ; then
     DOCKER=docker.io
   elif which docker >> /dev/null 2>&1 ; then
     DOCKER=docker
   else
     echo "Error: docker must be installed"
     exit 1
   fi
 
   $DOCKER -h 2> /dev/null | grep -q Jansens && { echo "Error: $DOCKER is the Docking System Tray - install docker.io instead"; exit 1; }
   $DOCKER version | grep -q "^Client version:" || { echo "Error: unexpected output from \"$DOCKER version\""; exit 1; }
   $DOCKER version | grep -q "^Server version:" || { echo "Error: could not get docker server version - check it is running and your permissions"; exit 1; }
 }
 
 # Construct a container and leave its name in $CONTAINER for future use.
 function initcontainer() {
   [ "$1" ] || { echo "Error: platform name must be specified"; exit 1; }
 
   DFILE="$ROOTDIR/contrib/docker/$1"
   [ -f "$DFILE" ] || { echo "Error: docker file $DFILE not found"; exit 1; }
 
   CONTAINER="hg-dockerrpm-$1"
   DBUILDUSER=build
   (
     cat $DFILE
     if [ $(uname) = "Darwin" ] ; then
         # The builder is using boot2docker on OS X, so we're going to
         # *guess* the uid of the user inside the VM that is actually
         # 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 -
 }