Hi @Damian
thanks for following up. I can confirm that the instance storage is large enough to run the full build. However, we are running the build inside of docker. Our ci scripts are designed to run (also) locally and for safety mark the build folder read-only. Modifications only happen in the docker container.
So I believe the issue here is that docker does not put its data files in the s390x travis instance storage. According to https://beta.docs.docker.com/engine/reference/commandline/dockerd/#miscellaneous-options this can be changed on startup of the docker daemon.