Allow to replace "app" server name in nginx by value of an env var

By default, nginx sends php requests to an upstream "app" server.
While keeping this default behavior, this commit allows the user
to override this server name at runtime, by setting an
APP_SERVERNAME env var to the nginx container

web-nginx/Dockerfile

@@ -2,4 +2,14 @@ FROM nginx:alpine
 
 HEALTHCHECK CMD curl --fail http://localhost/tt-rss/index.php || exit 1
 
-COPY nginx.conf /etc/nginx/nginx.conf
+COPY nginx.conf /etc/nginx/templates/nginx.conf.template
+
+# By default, nginx will send the php requests to "app" server, but this server
+# name can be overridden at runtime by passing an APP_SERVERNAME env var
+ENV APP_SERVERNAME=${APP_SERVERNAME:-app}
+
+# It's necessary to set the following NGINX_ENVSUBST_OUTPUT_DIR env var to tell
+# nginx to replace the env vars of /etc/nginx/templates/nginx.conf.template
+# and put the result in /etc/nginx/nginx.conf (instead of /etc/nginx/conf.d/nginx.conf)
+# See https://github.com/docker-library/docs/tree/master/nginx#using-environment-variables-in-nginx-configuration-new-in-119
+ENV NGINX_ENVSUBST_OUTPUT_DIR=/etc/nginx

web-nginx/nginx.conf

@@ -17,7 +17,7 @@ http {
 	index index.php;
 
 	upstream app {
-		server app:9000;
+		server ${APP_SERVERNAME}:9000;
 	}
 
 	server {