I created the fork of the UBSync with included arm64 support - It works, but it definitely needs a revision. I believe, that most of the steps are correct, but especially, the 32bit build needs a re-revision - it should be principally equal as the forked one - the same binaries (I only changed paths to enable multiarch).

Changes are described in README.

I had no luck with including the build process into the package itself because it went me into the dependency hell in the container (given probably by mixed repos in the default container - I'm not experienced enough with clickable and docker to deal with in reasonable time). I also experienced errors (Access Forbidden) with the ubports binaries of owncloudcmd extracted from deb packages. The same I experienced for some of older self-compiled owncloud versions. I finally build v2.5.3 in libertine, extracted binaries and it works.

The fork is now on my GitHub
The 64-bit click package is also there for testing