debian: add postinst script to configure sub UID/GID

debian/anbox.postinst

+#!/bin/sh
+# postinst script for anbox
+#
+# see: dh_installdeb(1)
+
+case "$1" in
+    configure)
+        usermod --add-subuids 100000-65536 phablet
+        usermod --add-subgids 100000-65536 phablet
+    ;;
+
+    abort-upgrade|abort-remove|abort-deconfigure)
+    ;;
+
+    *)
+        echo "postinst called with unknown argument \`$1'" >&2
+        exit 1
+    ;;
+esac
+
+# dh_installdeb will replace this with shell code automatically
+# generated by other debhelper scripts.
+
+#DEBHELPER#
+
+exit 0