Intermittent failure of update_image.py with Docker mount: cannot create regular file /mnt/boot/efi

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Sometimes when running xbstrap run make-image using Docker I get the following error:

$ xbstrap run make-image qemu
xbstrap: Running the following plan:
    run            update-initrd
    run            make-image
    run            qemu
xbstrap: run update-initrd [1/3]
xbstrap: Running ['@SOURCE_ROOT@/managarm/tools/gen-initrd.py', '--triple', '@OPTION:arch-triple@'] (tools: ['cross-binutils']) in Docker
xbstrap: run make-image [2/3]
xbstrap: Running ['@SOURCE_ROOT@/scripts/update-image.py', '--triple', '@OPTION:arch-triple@', '--mount-using', '@OPTION:mount-using@'] (tools: [])
update-image: Running the following plan:
 - mount
 - update file system
 - unmount
update-image: Running action mount ("image" to "mountpoint" using docker)
update-image: Running action update file system (from "system-root" to "mountpoint")
update-image: Updating the image doesn't require root
update-image: Update target has an EFI directory
update-image: Updating the file system...
+ mkdir -p /mnt/boot/managarm
+ install system-root/usr/managarm/bin/eir-stivale /mnt/boot/managarm
+ install -s --strip-program=tools/cross-binutils/bin/x86_64-managarm-strip system-root/usr/managarm/bin/eir-mb1 /mnt/boot/managarm
+ install -s --strip-program=tools/cross-binutils/bin/x86_64-managarm-strip system-root/usr/managarm/bin/thor /mnt/boot/managarm
+ install initrd.cpio /mnt/boot/managarm
+ mkdir -p /mnt/root
+ mkdir -p /mnt/usr/bin
+ mkdir -p /mnt/usr/lib
+ mkdir -p /mnt/var
+ mkdir -p /mnt/dev
+ mkdir -p /mnt/proc
+ mkdir -p /mnt/run
+ mkdir -p /mnt/sys
+ mkdir -p /mnt/tmp
+ mkdir -p /mnt/boot/grub
+ mkdir -p /mnt/home
+ cp /stuff/scripts/grub.cfg /mnt/boot/grub
+ cp /stuff/scripts/limine.cfg /mnt/boot/limine.cfg
+ sed -i 's|@ROOT_UUID@|4C722EAE-4DE0-1E4C-A5E6-8C5E0D62ED3E|g' /mnt/boot/limine.cfg
+ cp /stuff/scripts/limine.cfg /mnt/boot/efi/
cp: cannot create regular file '/mnt/boot/efi/': Not a directory
update-image: Action update file system failed with: ('Command "docker ..." failed with code 1',)
update-image: Running action unmount ("/mnt inside of container" using docker)
update-image: Finished running plan.
update-image: 1 step(s) failed.
xbstrap: Action run of task make-image failed

This appears to happen fairly consistently after a reboot, and goes away after re-building the image from scratch (with image_create.sh).