$ mount | grep binfmt_misc || mount -t binfmt_misc binfmt_misc /proc/sys/fs/binfmt_misc
I guess this mounting was somehow happening automatically before, but now it seems that it is handled by systemd in a user system. But a container usually doesn't run systemd.
Ok, your hint lead me to the fix:
$ mount | grep binfmt_misc || mount -t binfmt_misc binfmt_misc /proc/sys/ fs/binfmt_ misc
I guess this mounting was somehow happening automatically before, but now it seems that it is handled by systemd in a user system. But a container usually doesn't run systemd.