Hi
The original documentation you followed as far as I can tell has been provided by an end-user (cslaughter) who somehow managed to get this working on his synology nas unit.
I too am an end-user I don’t have a synology drive running the cms, however looking through the document and the comments afterwards, he states not to use the normal upgrade procedure but rather first to make sure you have a full backup, then go into the docker control panel and shutdown the old container, change some settings of the old container so they don’t clash with the new one, including renaming them, then install the new container using the original settings that you used 6 months ago, and fire up the new container (as far as i can tell this new one will still point to your original cms-db), however probably better reading the original posters comments yourself.
A link to his comment on upgrading and 3 responses from others relating to upgrading.
Just in case the link doesn’t work look for a comment in the instructions you followed, the comment is dated July 2017.
Hope that helps.
I’m just an end user too so very unlikely to be able to help further, cslaugher might though.
Bullfrog