github: add schedule for daily build

docs/release-notes/release-notes-0.15.0.md

@@ -45,6 +45,9 @@
   in review because of unmet contribution guideline
   requirements](https://github.com/lightningnetwork/lnd/pull/6080).
 
+* [A nightly build of the `lnd` docker image is now created
+  automatically](https://github.com/lightningnetwork/lnd/pull/6160).
+
 ## RPC Server
 
 * [Add value to the field