Example run here: 2022-05-17-v5 · commonknowledge/leftbookclub@b8306be · GitHub
Whenever a docker layer can’t be reused from cache, this ‘no space left’ error throws.
My guess is this is something to do with the remote builder, but I have no idea how to debug this.
A common cause of the no-space error when using a remote builder is indeed its storage is full. Since volumes don’t auto-resize, a temporary solution is simply to destroy your remote builder. A new one will then be made on the next deploy, with a new disk, and so will then have space 
Run fly apps list to see the name of your builder app (if you don’t see it from your log/error).
Run fly destroy the-name-of-the-remote-builder to destroy the remote builder.
And then try another deploy. And see if that fixes it. May be as simple as that.
That did the trick! Thanks very much. I might put this on some kind of cron…