feat: write current process id to all acquired lock files

This allows us to check on what process created a lock.

Additionally, we now always have a DOKKU_PID value, even when running in cron (which does not have access to BASH_PID.

plugins/common/functions

@@ -983,6 +983,7 @@ acquire_advisory_lock() {
       dokku_log_fail "Run 'apps:unlock' to release the existing deploy lock"
     fi
   fi
+  echo "$DOKKU_PID" >"$LOCK_FILE"
 }
 
 release_advisory_lock() {