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