Docker:latest cannot be pulled

We’ve setup two runners “the day before” - which is Thu 22 2018 and connected them to our gitlab project. The runners are installed in raspberry PI with a blank raspbian os from 3/13/2018. The docker executor mode is used, so we’ve also installed docker. yesterday, our complete pipeline was executed, all based on docker executor.

Today we setup a third runner and the trouble began:

pi@samson:~ $ docker pull “docker:latest”
latest: Pulling from library/docker
no matching manifest for linux/arm in the manifest list entries
-> we’re really sure this was working yesterdat and docker:latest also seems to support arm32v6. strange.

#2 we’ve switched to arm32v6/docker. we can pull that image manually, but if the runner starts the job which is based on this image, it tells us:
WARNING: Job failed: Error response from daemon: pull access denied for arm32v6, repository does not exist or may require ‘docker login’ job=59075395 project=5826161 runner=c53117f9
-> we’ve tried to login to our dockerhub account, does not help.

does anyone have an idea what could be wrong?