I'm not sure how to make strace pid 1 :-) And I'm not quite ready to abandon Debian yet. I've created a virt-install script which reproduces the problem reliably (honestly all it does is install jessie, then lxc-docker). Hopefully my workload eases and I can put this trivially reproducible thing into a bug report somewhere (debian? systemd? docker) for somebody to see what's going on.
FWIW systemctl start docker works fine after the system has booted. It's configured to require network.target, and even though I can see network target scroll by during boot, it's still not being noticed.
The work-around is to make multiuser want docker, then I've got docker starting at bootup.
Sounds like lxc-docker.service simply wasn't enabled. "enabling" is basically creating a symlink from multi-user.target.wants to lxc-docker.service. This is done by 'systemctl enable lxc-docker.service' once that service declares WantedBy=multi-user.target in its [Install] section.
Other symptoms that would indicate this was the problem would have been that 'systemctl status lxc-docker.serivce' would have said "Loaded: loaded (...; disabled)" and "Active: inactive (dead)". i.e., systemd never started the service because nothing ever wanted it.
strace -p1 should do the trick, you can attach after start, but you will have to get it started early, probably as a systemd service. Or if its in a container, strace form outside the container.
FWIW systemctl start docker works fine after the system has booted. It's configured to require network.target, and even though I can see network target scroll by during boot, it's still not being noticed.
The work-around is to make multiuser want docker, then I've got docker starting at bootup.