Persist to workspace suddenly broken in CI 2.0

Last time I changed this file was 16 days ago, today it started failing to copy to persist storage between jobs:

- persist_to_workspace:
          root: ~/repo
            - build


Digest: sha256:34aa80bb22c79235d466ccbbfa3659ff815100ed21eddb1543c6847292010c4d
Status: Downloaded newer image for nginx:1.13-alpine
 ---> 22f5726c6dc0
Step 2/3 : COPY build /app
INFO[0002] COPY failed: stat /var/lib/docker/tmp/docker-builder359448335/build: no such file or directory 
Exited with code 1