[1/17/2019]
docker-ce=18.09 segfaults on RPi. getflat script has changed to downgrade to 18.06.1.
[1/1/2019]
We confirmed docker-ce=18.09 no longer has the bug we discussed below, so the recommended downgrade is no more necessary, but we will keep this record for the future just in case.
[5/6/2018]
We found docker-ce=18.04.0~ce has bug on RPi ('docker *' commands hang after system restart; some 'docker pull' fails with invalid EOF during untar, & etc.), and if that happens to you too, DOWNGRADE to 17.12.1~ce or lower:
1. Find out all available version in your checked out apt repository
sudo apt-cache policy docker-ce
2. Install older docker-ce
sudo apt-get install docker-ce=17.12.1~ce-0~raspbian
-'getflat' automatically downgrade it in this method.