Someone tried to compile that for this platform? zLinux!
There’s an IBM team working on Power and Z support - for eg https://github.com/docker/docker/pull/9715
In case you hadn’t already found it, pre-built Docker binaries for the Z architecture for RHEL7 and SLES12 are available as well.
http://www.ibm.com/developerworks/linux/linux390/docker.html