It means the kernel is 64 bit, but the OS is 32bit, which includes all the packages, services and processes, including the docker service (that's why I'm surprised the user reported a 64 bit container worked). I mean the container does use the host's kernel so a 64bit kernel would definitely be a requirement, but I would have guessed other docker service related activities would need 64 bit in the userspace as well. I wonder if it's one of those things where some containers work fine, but some don't.
We need some testing before I do a mass update of all the addons.