I found a question on Stack Overflow where someone else is running into the same problem with Docker on Debian 11: https://stackoverflow.com/questions/69002675/. In my case, the Linux system where /proc/self/cgroup is returning the correct information is running Debian 10, and the one where it isn’t is running Debian 11. It looks like there may be ways to work around the change in Debian 11, but for now I’ll switch back to Debian 10.
Edit: After a bit more digging, I found that the container ID is still exposed in /proc/self/mountinfo:
user@debian:~$ docker run --entrypoint /bin/bash --rm -it -v /deploy -v $(pwd):/workdir -v storage:/storage --net=host -v /var/run/docker.sock:/var/run/docker.sock torizon/torizoncore-builder:3.1.0
root@debian:/workdir# cat /proc/self/mountinfo
587 430 0:42 / / rw,relatime master:222 - overlay overlay rw,lowerdir=/var/lib/docker/overlay2/l/7UXPC4DMWOMAKZOPT2AU5TO42F:/var/lib/docker/overlay2/l/N23BUIPF6JSDAUEXPF3OVGNH63:/var/lib/docker/overlay2/l/WBCQ5TMZHES7JVXWO47UM55YG5:/var/lib/docker/overlay2/l/J43KCBC7576AQRYPCDWBHJ5PIA:/var/lib/docker/overlay2/l/IQSRCWDU7ARSW5SSYAA2MYR2ZT:/var/lib/docker/overlay2/l/PPCZZPQHTCQ4IW75U62SOSZJZ4:/var/lib/docker/overlay2/l/BL5EDRXDUHXPNG5ECDMVSUU6LP:/var/lib/docker/overlay2/l/O2MR4SA67GGKA3BX6SXBQNET5P:/var/lib/docker/overlay2/l/SYK2HVX6QTSMN3LTSEMC6MEK2V:/var/lib/docker/overlay2/l/A77ZL2BP77DWS5QH3RFNNVP4BP:/var/lib/docker/overlay2/l/NWBBA52E5JI2343JBIZOO7MEC4:/var/lib/docker/overlay2/l/WK47J7O3FBS6S2KA4CHI2AOX76:/var/lib/docker/overlay2/l/SCN6UKOU6DKEDFUAK3XY3ZD5VA:/var/lib/docker/overlay2/l/ZTTWJGFWJPK7E4PSRI7WU6B4IK:/var/lib/docker/overlay2/l/76JGUU6DDVGMCBLG65OKGPX6QV:/var/lib/docker/overlay2/l/GHNOJFO7OIQ5LZI3JDAUD4K2EP:/var/lib/docker/overlay2/l/B3VAX4JLENH4BJV635U7UBDJRC,upperdir=/var/lib/docker/overlay2/f790066af0926dde260f6d436f8bf1bbb4449fdf8bc1c74a38bc7c06b4be927f/diff,workdir=/var/lib/docker/overlay2/f790066af0926dde260f6d436f8bf1bbb4449fdf8bc1c74a38bc7c06b4be927f/work
588 587 0:57 / /proc rw,nosuid,nodev,noexec,relatime - proc proc rw
589 587 0:58 / /dev rw,nosuid - tmpfs tmpfs rw,size=65536k,mode=755
590 589 0:60 / /dev/pts rw,nosuid,noexec,relatime - devpts devpts rw,gid=5,mode=620,ptmxmode=666
591 587 0:20 / /sys ro,nosuid,nodev,noexec,relatime - sysfs sysfs rw
592 591 0:27 / /sys/fs/cgroup ro,nosuid,nodev,noexec,relatime - cgroup2 cgroup rw
593 589 0:56 / /dev/mqueue rw,nosuid,nodev,noexec,relatime - mqueue mqueue rw
594 589 0:61 / /dev/shm rw,nosuid,nodev,noexec,relatime - tmpfs shm rw,size=65536k
595 587 8:1 /home/user /workdir rw,relatime - ext4 /dev/sda1 rw,errors=remount-ro
596 587 8:1 /var/lib/docker/volumes/c90738bc179ec28d104cf44fb54625ff35efa0ff782508837cb5b41de5055cf9/_data /deploy rw,relatime master:1 - ext4 /dev/sda1 rw,errors=remount-ro
597 587 8:1 /var/lib/docker/volumes/storage/_data /storage rw,relatime master:1 - ext4 /dev/sda1 rw,errors=remount-ro
598 587 8:1 /var/lib/docker/containers/edaf7ad2c9a2c3b64ae9b22c9faeb2245f7ededd71ae44caab23e964f1a65769/resolv.conf /etc/resolv.conf rw,relatime - ext4 /dev/sda1 rw,errors=remount-ro
599 587 8:1 /var/lib/docker/containers/edaf7ad2c9a2c3b64ae9b22c9faeb2245f7ededd71ae44caab23e964f1a65769/hostname /etc/hostname rw,relatime - ext4 /dev/sda1 rw,errors=remount-ro
600 587 8:1 /var/lib/docker/containers/edaf7ad2c9a2c3b64ae9b22c9faeb2245f7ededd71ae44caab23e964f1a65769/hosts /etc/hosts rw,relatime - ext4 /dev/sda1 rw,errors=remount-ro
601 587 0:23 /docker.sock /run/docker.sock rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,size=302752k,mode=755
431 589 0:60 /0 /dev/console rw,nosuid,noexec,relatime - devpts devpts rw,gid=5,mode=620,ptmxmode=666
432 588 0:57 /bus /proc/bus ro,relatime - proc proc rw
433 588 0:57 /fs /proc/fs ro,relatime - proc proc rw
434 588 0:57 /irq /proc/irq ro,relatime - proc proc rw
445 588 0:57 /sys /proc/sys ro,relatime - proc proc rw
495 588 0:57 /sysrq-trigger /proc/sysrq-trigger ro,relatime - proc proc rw
496 588 0:62 / /proc/asound ro,relatime - tmpfs tmpfs ro
512 588 0:63 / /proc/acpi ro,relatime - tmpfs tmpfs ro
513 588 0:58 /null /proc/kcore rw,nosuid - tmpfs tmpfs rw,size=65536k,mode=755
526 588 0:58 /null /proc/keys rw,nosuid - tmpfs tmpfs rw,size=65536k,mode=755
540 588 0:58 /null /proc/timer_list rw,nosuid - tmpfs tmpfs rw,size=65536k,mode=755
541 588 0:58 /null /proc/sched_debug rw,nosuid - tmpfs tmpfs rw,size=65536k,mode=755
542 591 0:64 / /sys/firmware ro,relatime - tmpfs tmpfs ro
root@debian:/workdir#
And from another terminal:
user@debian:~$ docker ps --no-trunc
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
edaf7ad2c9a2c3b64ae9b22c9faeb2245f7ededd71ae44caab23e964f1a65769 torizon/torizoncore-builder:3.1.0 "/bin/bash" 10 seconds ago Up 10 seconds elated_kirch
b12ac3a8d6cd51ebb82968f3d1c3491113df47f7e56839411351d57c2ad5b213 registry:2 "/entrypoint.sh /etc/docker/registry/config.yml" 2 months ago Up 45 hours 0.0.0.0:5000->5000/tcp registry
user@debian:~$
I’m not sure if this is guaranteed to always work this way, though.