nuke-graph-directory.sh error on btrfs subvolume delete

[brettmilford]

Description
nuke-graph-directory.sh errors out at L66 of:

moby/contrib/nuke-graph-directory.sh

Lines 59 to 70 in 2acab35

	if command -v btrfs > /dev/null 2>&1; then
	# Find btrfs subvolumes under $dir checking for inode 256
	# Source: http://stackoverflow.com/a/32865333
	for subvol in $(find "$dir" -type d -inum 256 | sort -r); do
	if [ "$dir" != "$subvol" ]; then
	(
	set -x
	btrfs subvolume delete "$subvol"
	)
	fi
	done
	fi

On non-btrfs systems. As this is called by post rm hooks in apt packaging the package fails to uninstall.

Specifically on my system which is using zfs; btrfs tools are installed by default, thus:

if command -v btrfs > /dev/null 2>&1; then

works, and

find "$dir" -type d -inum 256

returns a number of directories which are not btrfs volumes on my system.

Steps to reproduce the issue:
On ubuntu 20.10

1. sudo apt-get install docker.io
2. sudo apt-get purge docker.io

Describe the results you received:

Purging configuration files for docker.io (19.03.13-0ubuntu3) ...

Nuking /var/lib/docker ...
  (if this is wrong, press Ctrl+C NOW!)

+ sleep 10
ok
+ btrfs subvolume delete /var/lib/docker/containers/242b750a0952a99008adc6cd37f749423eeacc6c92dce2e3a509350977a41a8c
ERROR: Not a Btrfs filesystem: Invalid argument
dpkg: error processing package docker.io (--purge):
 installed docker.io package post-removal script subprocess returned error exit status 1
Errors were encountered while processing:
 docker.io
E: Sub-process /usr/bin/dpkg returned an error code (1)

Describe the results you expected:
Command succeeds.