Add option to support updating image on start/restart

[DrewBarrett]

Basically I'm proposing to add a flag that changes the unit file to contain

TimeoutStartSec=0
ExecStartPre=-/usr/bin/docker kill image
ExecStartPre=-/usr/bin/docker rm image
ExecStartPre=/usr/bin/docker pull user/image

as opposed to the current

ExecStartPre=-{{ docker_path }} rm -f {{ container_name }}

I'm going to implement this for my own use, so let me know if you'd like me to submit a pull request.

[mhutter]

What's the purpose behind this change? The point of docker rm CONTAINERname is to avoid conflicts when running docker run.

To update the image before (re)starting, only ExecStartPre=/usr/bin/docker pull user/image is required.

[DrewBarrett]

Yeah that was just an extra verbose example (and rm image was a typo for rm containername. I believe TimeoutStartSec=0 is also needed in the case of large images so it does not hit the default systemd timeout