packaging: add `HG_DOCKER_OWN_USER` to `dockerdeb` like exists in `dockerrpm`

I was getting build failures when it was trying to write to the working
directory on CentOS 7 without this.  It is basically the same as was added to
the RPM builder in 4c0d4bbdc395.

For some reason, this doesn't work with Xenial, and the only solution I found
was to invoke it with UID 1000 on the host side.  It doesn't EOL until April
2024, but it also has python 3.5.2, so this build complication is the least of
the problems with supporting it after py2 is dropped.

Differential Revision: https://phab.mercurial-scm.org/D9394

contrib/packaging/dockerdeb

@@ -1,40 +1,44 b''
 #!/bin/bash -eu
 
 . $(dirname $0)/packagelib.sh
 
 BUILDDIR=$(dirname $0)
 export ROOTDIR=$(cd $BUILDDIR/../.. > /dev/null; pwd)
 
 DISTID="$1"
 CODENAME="$2"
 PLATFORM="$1-$2"
 shift; shift # extra params are passed to build process
 
 OUTPUTDIR=${OUTPUTDIR:=$ROOTDIR/packages/$PLATFORM}
 CONTAINER=hg-docker-$PLATFORM
 
 DOCKER=$($BUILDDIR/hg-docker docker-path)
 
 $BUILDDIR/hg-docker build \
     --build-arg CODENAME=$CODENAME \
     $BUILDDIR/docker/$DISTID.template \
     $CONTAINER
 
 # debuild only appears to be able to save built debs etc to .., so we
 # have to share the .. of the current directory with the docker
 # container and hope it's writable. Whee.
 dn=$(basename $ROOTDIR)
 
+if [[ -z "${HG_DOCKER_OWN_USER:-}" ]]; then
 DBUILDUSER=build
+else
+    DBUILDUSER="$(id -u):$(id -g)"
+fi
 
 if [ $(uname) = "Darwin" ] ; then
     $DOCKER run -u $DBUILDUSER --rm -v $PWD/..:/mnt $CONTAINER \
             sh -c "cd /mnt/$dn && make clean && make local"
 fi
 $DOCKER run -u $DBUILDUSER --rm -v $ROOTDIR/..:/mnt $CONTAINER \
   sh -c "cd /mnt/$dn && DEB_BUILD_OPTIONS='${DEB_BUILD_OPTIONS:=}' contrib/packaging/builddeb --build --distid $DISTID --codename $CODENAME $@"
 (cd $ROOTDIR && contrib/packaging/builddeb --cleanup --distid $DISTID --codename $CODENAME)
 if [ $(uname) = "Darwin" ] ; then
     $DOCKER run -u $DBUILDUSER --rm -v $PWD/..:/mnt $CONTAINER \
             sh -c "cd /mnt/$dn && make clean"
 fi