new: provide ``$DOCKER_HOST_NET`` and ``$DOCKER_HOST_IP``.

bin/compose

@@ -2007,6 +2007,14 @@ fi
 _setup_state_dir
 mkdir -p "$CACHEDIR" || exit 1
 
+DOCKER_HOST_NET=$(ip addr show docker0 | grep 'inet ' | xargs echo | cut -f 2 -d " ") || {
+    die "Couldn't determine docker host net"
+}
+DOCKER_HOST_IP=$(echo "$DOCKER_HOST_NET" | cut -f 1 -d "/")  || {
+    die "Couldn't determine docker host ip"
+}
+export DOCKER_HOST_NET DOCKER_HOST_IP
+
 ##
 ## Argument parsing
 ##