Thanks for taking the time to file a bug!
The issue looks to be:
addgroup: The group `docker' already exists and is not a system group.
If you type 'apt install -f' the install should complete just fine.
** Changed in: docker.io (Ubuntu)
Status: New => Incomplete
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package docker.io 1.13.1-0ubuntu1~16.04.2 failed to install/upgrade:
subprocess installed post-installation script returned error exit
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