I am having the same issue upgrading from 1.11.2-beta15 to 1.12.0-rc2-beta16. I have been able to upgrade in the past, but as soon as we transitioned to docker 1.12 the updates stopped working
Checking for updates offers to download and install 1.12.0-rc2-beta16, but after install and relaunch, notification window says “Docker has been updated! New version is 1.11.2-beta15” but that’s what I already had.