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

@@ -25,7 +25,11 b' DOCKER=$($BUILDDIR/hg-docker docker-path'
 # container and hope it's writable. Whee.
 dn=$(basename $ROOTDIR)
 
-DBUILDUSER=build
+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 \