Comments on the debdiff:

diff -Nru docker.io-17.03.2/debian/docker.io.config docker.io-17.03.2/debian/docker.io.config
--- docker.io-17.03.2/debian/docker.io.config   1970-01-01 00:00:00.000000000 +0000
+++ docker.io-17.03.2/debian/docker.io.config   2018-03-23 02:26:26.000000000 +0000
@@ -0,0 +1,11 @@
+#!/bin/sh
+set -e
+
+. /usr/share/debconf/confmodule
+
+db_version 2.0
+
+db_input high docker.io/restart || :
+db_go
+
+db_stop

This config script has the properties that:
 - the first time the question is asked, it will (probably) be shown to the user.
 - on subsequent invocations of the package's maintainer scripts, the question will not be shown.

This means that, once the user has answered the question, this behavior will be sticky; every subsequent update of the package will run the risk of either leaving the tools unable to talk to the daemon, or unexpectedly stopping (and failing to restart) containers, resulting in a service interruption.

The reason this is being done as a debconf question is because neither option is good.  And *since* we are forced to ask the user to pick their poison, I consider it insufficient to ask this question only once. The question should by default be re-asked, at each package upgrade, so that the user has an opportunity to make the decision that is correct for them at that moment.

The pattern for this is:
 - no .config script
 - call 'db_fset docker.io/restart seen false' in postinst
 - call 'db_input high docker.io/restart' in postinst only if a restart may be required

It would be ok to give the user an additional option to "always restart" (so, a tristate question instead of a boolean); but at minimum users must be able to select the behavior locally at the point of any given upgrade.

In addition, the current default for the debconf question is "true"; so by default, which includes on non-interactive upgrades (unattended upgrades or the like), the behavior the user gets here is that the service is restarted, stopping any un-supervised containers.  If the docker.io package is not able to detect whether all running containers will be automatically restarted, then if this is to be a supported use case at all, the default should be to not restart: because it is less disruptive to have the docker service become unavailable, than to have the docker containers become unavailable.

I am therefore rejecting the upload of 17.03.2-0ubuntu3~16.04.1.

Additional observations about the package, which are not blockers for an SRU:

+shouldStartRestart=
+if [ "$1" != 'reconfigure' ] && [ -z "${DEBCONF_RECONFIGURE:-}" ]; then
+       # (if we're just doing "dpkg-reconfigure", there's no reason to actually [re]start Docker)
+       shouldStartRestart=1
+fi

"reconfigure" is not a valid argument to a postinst script.  (And dpkg-reconfigure does not invoke it this way - it invokes it as 'postinst configure'.)  So the first part of this test is always true and can be omitted.

+if \
+       [ "$RET" = 'true' ] \
+       && [ -n "$2" ] && [ "$1" = 'configure' ] \
+       && [ -n "$shouldStartRestart" ] \
+       && [ -x /etc/init.d/docker ] \
+       && invoke-rc.d --disclose-deny docker status > /dev/null 2>&1 \

The last two checks are not idiomatic.  These may appear in debhelper autogenerated snippets, but those need to account for a lot of corner cases which shouldn't apply here.  There should be no need to check for /etc/init.d/docker's presence/executability as it should always exist (anyone trying to disable a service by marking the init script not executable is off the reservation).  And the check for status is not consistent with expected behavior of packages: upgrading the package should normally start the service even if it was currently not running, unless the administrator has explicitly disabled it through policy-rc.d.

+# because we had to use "dh_installinit --no-start", we get to make sure the daemon is started on first install
+# (this is adapted from debhelper's "autoscripts/postinst-init")
+if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ] || [ "$1" = "abort-deconfigure" ] || [ "$1" = "abort-remove" ]; then
+       if [ -x /etc/init.d/docker ] && [ -n "$shouldStartRestart" ] && ! invoke-rc.d docker status > /dev/null 2>&1 ; then
+               invoke-rc.d docker start || :
+       fi
+fi

This code includes a check for "$shouldStartRestart"; but this is the code to start the daemon on package initial installation, and the debconf question doesn't ask about starting, only about restarting.  So I think this check shouldn't be there.