if the docker daemon wasn't already running, your package install fails

[StefanScherer]

FYI if the docker daemon wasn't already running, your package install fails. You should probably put || true after the invoke-rc.d stop in your preinstall bit:

(Reading database ... 54022 files and directories currently installed.)
Preparing to replace docker-hypriot 1.5.0-7 (using docker-hypriot_1.6.0-1_armhf.deb) ...
[....] Stopping Docker: dockerstart-stop-daemon: warning: failed to kill 1745: No such process
1 pids were not killed
No process in pidfile '/var/run/docker-ssd.pid' found running; none killed.
invoke-rc.d: initscript docker, action "stop" failed.
dpkg: warning: subprocess old pre-removal script returned error exit status 1
dpkg: trying script from the new package instead ...
[....] Stopping Docker: dockerstart-stop-daemon: warning: failed to kill 1745: No such process
1 pids were not killed
No process in pidfile '/var/run/docker-ssd.pid' found running; none killed.
invoke-rc.d: initscript docker, action "stop" failed.
dpkg: error processing docker-hypriot_1.6.0-1_armhf.deb (--install):
subprocess new pre-removal script returned error exit status 1
dpkg: error while cleaning up:
subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
docker-hypriot_1.6.0-1_armhf.deb
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