Update page 'Home'

Home.md

@@ -62,6 +62,39 @@ volumes:
 
 Copy and/or git clone any third party plugins into ``plugins.local`` as usual.
 
+### I'm running into 502 errors and/or other connectivity issues
+
+First, check that all containers are running:
+
+```
+$ docker-compose ps
+                   Name                                 Command               State           Ports
+------------------------------------------------------------------------------------------------------------
+ttrss-docker-demo_app_1_f49351cb24ed         /bin/sh -c /startup.sh           Up      9000/tcp
+ttrss-docker-demo_backups_1_8d2aa404e31a     /dcron.sh -f                     Up      9000/tcp
+ttrss-docker-demo_db_1_fc1a842fe245          docker-entrypoint.sh postgres    Up      5432/tcp
+ttrss-docker-demo_updater_1_b7fcc8f20419     /updater.sh                      Up      9000/tcp
+ttrss-docker-demo_web-nginx_1_fcef07eb5c55   /docker-entrypoint.sh ngin ...   Up      127.0.0.1:8280->80/tcp
+```
+
+Then, ensure that frontend (`web-nginx`) container is up and can contact FPM (`app`) container:
+
+```
+$ docker-compose exec web-nginx ping app
+PING app (172.18.0.3): 56 data bytes
+64 bytes from 172.18.0.3: seq=0 ttl=64 time=0.144 ms
+64 bytes from 172.18.0.3: seq=1 ttl=64 time=0.128 ms
+64 bytes from 172.18.0.3: seq=2 ttl=64 time=0.206 ms
+^C
+--- app ping statistics ---
+3 packets transmitted, 3 packets received, 0% packet loss
+round-trip min/avg/max = 0.128/0.159/0.206 ms
+```
+
+Containers communicate via DNS names assigned by Docker based on service names defined in `docker-compose.yml`. This means that services (specifically, `app`) in the YML must not be renamed, and Docker DNS service should be functional.
+
+Similar issues may be also caused by Docker `iptables` functionality either being disabled or conflicting with `nftables`.
+
 ### How do I put this container behind a reverse proxy?
 
 A common pattern is shared nginx doing SSL termination, etc. Don't forget to pass `X-Forwarded-Proto` to the container if you're using HTTPS, otherwise tt-rss would generate plain HTTP URLs.