@aknobbe try using administration/jenkins/win32_build/compile_client.sh at master · owncloud/administration · GitHub
rather than https://github.com/owncloud/client/blob/master/admin/win/docker/build.sh. I think that script might be a bit outdated/incomplete. I wrote some instructions in the past here:
P.S. Are you trying to apply some theming? maybe something there is causing makensis to fail - what does NSISOutput.log say?