Add Onion-Location header for Tor Browser discovery.

Mirror NullCart's ONION_HOSTNAME env and nginx clearnet header so visitors on HTTPS see ".onion available".

nginx/docker-entrypoint.sh

@@ -2,7 +2,14 @@
 set -eu
 
 export CLEARNET_DOMAIN
-envsubst '${CLEARNET_DOMAIN}' \
+
+if [ -n "${ONION_HOSTNAME:-}" ]; then
+    export ONION_LOCATION_HEADER="add_header Onion-Location \"http://${ONION_HOSTNAME}\$request_uri\";"
+else
+    export ONION_LOCATION_HEADER=""
+fi
+
+envsubst '${CLEARNET_DOMAIN} ${ONION_LOCATION_HEADER}' \
     < /etc/nginx/templates/conf.d/clearnet.conf.template \
     > /etc/nginx/conf.d/clearnet.conf