Ability to pull new image before starting

[benoist]

Currently for accessories, a reboot will stop the container, then start it with the new image causing a new download if the image version is updated.

To minimise downtime and errors if the images doesn't exist, it would be great if the accessory boot and reboot pull the image first, halting on errors.

[mike-weiner]

@benoist - I've opened #1626 to address this.