dns.env file did not work out, env_file: element not being read before dns: element. Using folder-specific .env files instead, seems to be loaded before dns: element. Also move other values to the env files for better password privacy.

Config/OnlyOffice/.env.example

@@ -0,0 +1,10 @@
+# Environment file for OnlyOffice.
+
+# Secret that should be typed into the Nextcloud Admin Settings UI.
+JWT_SECRET=abc123
+
+# If you have both Nextcloud and OnlyOffice on the same internal network you
+#   will likely want to set this to an internal DNS server. Some routers will
+#   drop traffic if an internal IP tries to communicate with the WAN IP, causing
+#   the curl to OnlyOffice's /healthcheck to fail. Do the same in both configs.
+DNS=192.168.1.1

Config/OnlyOffice/docker-compose.yml

@@ -11,10 +11,5 @@ services:
     ports:
       - 8000:80
       - 4443:443
-    environment:
-      - JWT_SECRET=abc123
-    env_file:
-      - ../../dns.env
     dns:
-      - $DNS1
-      - $DNS2
+      - "$DNS"