Dockerfile: support the PORT variable, expose 5000 by default

Dockerfile

@@ -8,5 +8,7 @@ ARG NODE_ENV=production
 RUN yarn build
 
 FROM nginx:stable-alpine
-COPY installation/docker.conf /etc/nginx/conf.d/default.conf
+EXPOSE 5000
+ENV PORT=5000
+COPY installation/docker.conf /etc/nginx/templates/default.conf.template
 COPY --from=build /app/static /usr/share/nginx/html

installation/docker.conf

@@ -1,5 +1,7 @@
 # Soapbox Nginx for Docker.
 server {
+  listen ${PORT};
+
   keepalive_timeout    70;
   sendfile             on;
   client_max_body_size 80m;