Just got this new error when trying to deploy to a Dokku box:
remote: app is currently being deployed or locked. Waiting...
The app had already failed and produced an error in the docker logs. The container was already stopped. However it seems Dokku wasn't aware. Removing the stopped containers didn't help. I did a little investigating and it appears Dokku locks by creating an empty hidden file:
I simply deleted this file and I was able to deploy again.