I assume you used a privileged container, I can't reproduce this with an unprivileged one with only CAP_SYS_ADMIN added.
After some digging, the issue appears to come from here: https://salsa.debian.org/installer-team/debootstrap/-/blob/master/functions#L1179 > umount "$TARGET/proc" 2>/dev/null || true As far as git can tell, this is an ancient line. As $TARGET/proc is a symlink to /proc, the umount usually fails with a "target is busy" error, which is ignored. In a privileged container however, one can umount /proc and render the system unusable. Is this line even needed in the first place? If it can't be removed, it should at least only be executed if $TARGET/proc isn't a symlink to /proc. I don't see a scenario where unmounting hosts /proc would be valid.
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