Ulimit is inheritance from its parent process and the parent is still the default for open files, 1024 (from the first init to your current shell, unless you made changes by adding it to the ~/.profile for the current session only or globally in the initramfs by rebuilding the firmware for ADM - other systems from /etc/security/limits.conf which is not implemented in ADM):
I personally don't use that script and I'm still seeing 1024 for open files in the latest 4.2.1.RGE2. You can revert by disabling the boot script and removing or commenting out the ulimit line which was added by the boot script to the start-stop.sh, then restart the docker-ce or just reboot. Note, the K50docker-ce and S50docker-ce are just symlinks to the start-stop.sh, so just modify any one of them or start-stop.sh itself.
ps: I'm looking at an unexpected first time encounter issue with the 4.2.1.RGE2. The system crashed not responding after few days upgrading during a midnight heavy load backup period. Had few "system" mods done; apps running on external USB SSD, read caching to external USB SSD, raid5 on external USB, OS running in ram and built some kernel modules for ipset/ip6tables/USBIP. All these has been working flawlessly until this 4.2.1.RGE2. Due to how ADM works, the logs gets lost when I had to hard restart, so I had to do some tweaks to try catch the crash next time. Will post on a different thread if necessary.