Trust the active Docker network gateway

docker/entrypoint.sh

@@ -3,9 +3,9 @@
 set -eu
 
 if [ "${TRUST_HOST_PROXY:-0}" = "1" ]; then
-    proxy_ip="$(getent ahostsv4 host.docker.internal | sed -n '1{s/[[:space:]].*//;p;}')"
+    proxy_ip="$(ip -4 route show default | sed -n 's/^default via \([^ ]*\).*/\1/p' | head -n 1)"
     if [ -z "$proxy_ip" ]; then
-        echo "Cannot resolve the trusted Docker host gateway." >&2
+        echo "Cannot determine the trusted Docker network gateway." >&2
         exit 1
     fi
     export FORWARDED_ALLOW_IPS="127.0.0.1,$proxy_ip"