Thanks to lots of help from @cniweb, @wborn, @martinvw, @Benjy, and @andrey-yantsen the OH Docker Images will now automatically perform an upgrade on your userdata folder for you. No longer do we have to use the manual process of letting the container build a new userdata and copying our configs back over. Upgrading is now as simple as it is for all other installation methods.
The way it works is if userdata/etc/version.properties is different between the Container and the userdata folder mapped into the container it will assume that an upgrade needs to be performed.
First, it will create a full backup of your entire userdata folder, placing it as a dated tar file into userdata/backup.
Next, it copies the necessary files to userdata/etc, the same as apt-get/yum/upgrade script does.
Finally, it clears the cache and tmp.
In short, except for doing the backup differently and not redownloading OH itself, it does the same steps as the upgrade script.
This means users can now take advantage of tools like Watchtower to keep their openHAB Image up to date. It will also aid those who like to keep up with the nightly builds.
It should work from any 2.0 version forward and of course, if something goes wrong you have the full backup of your userdata folder.
It was just merged in so this should become available in the next build on dockerhub.
NOTE: A PR with updated upgrade instructions has been submitted to openhab-docs.