

It looks like you’re relying on media automounting to access the drive, but this is happening too late for Docker.
I would suggest creating the empty folder and explicitly adding the mount to /etc/fstab
instead. This should mount early enough, and even if it doesn’t it needs an empty folder for the mount point anyway.
Edit: Make sure you reference the partition by UUID, because the device name of USB devices sometimes change after a reboot.
Running copper between different buildings like that is risky, because if they have different ground potential you could get current traveling through the cable and zapping the equipment. A lot of people use fiber for those runs for that reason.
If you want to solve your issue as cheaply as possible while eliminating the aforementioned risk, move AP2 to where AP1 is and replace AP2 with a PoE-powered access point. This will ensure all devices are powered from the same house, and you get rid of the failing device.