Docker: open fallback backend at :4444

Dockerfile

@@ -10,7 +10,8 @@ RUN yarn build
 FROM nginx:stable-alpine
 EXPOSE 5000
 ENV PORT=5000
-ENV BACKEND_URL=http://localhost
+ENV FALLBACK_PORT=4444
+ENV BACKEND_URL=http://localhost:4444
 ENV CSP=
 COPY installation/docker.conf.template /etc/nginx/templates/default.conf.template
 COPY --from=build /app/static /usr/share/nginx/html

installation/docker.conf.template

@@ -18,8 +18,10 @@ map $http_accept $activitypub_location {
 
 proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=CACHE:10m inactive=7d max_size=1g;
 
+# Fake backend for when BACKEND_URL isn't defined.
 server {
-  server_name localhost;
+  listen ${FALLBACK_PORT};
+  listen [::]:${FALLBACK_PORT};
 
   location / {
     add_header Content-Type "application/json" always;