purging docker.io fails due to bash-specific "shopt" command

[Paul]

running "apt purge docker.io" fails. This is because /var/lib/docker/nuke-graph-directory.sh uses "/bin/sh" in the shebang line, yet it uses bash-specific "shopt" builtin.

updating the script to use "/bin/bash" fixes the issue and allows "apt purge" to finish successfully.

Below is the output from "apt purge docker.io"

hostname:~# apt purge docker.io
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be REMOVED:
  docker.io*
0 upgraded, 0 newly installed, 2 to remove and 72 not upgraded.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] y
(Reading database ... 72075 files and directories currently installed.)
Removing docker.io (1.13.1-0ubuntu1~16.04.2) ...
Purging configuration files for docker.io (1.13.1-0ubuntu1~16.04.2) ...

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

+ sleep 10

/var/lib/docker/nuke-graph-directory.sh: 64: /var/lib/docker/nuke-graph-directory.sh: shopt: not found
dpkg: error processing package docker.io (--purge):
 subprocess installed post-removal script returned error exit status 127
E: Sub-process /usr/bin/dpkg returned an error code (1)

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: docker.io 1.13.1-0ubuntu1~16.04.2
ProcVersionSignature: Ubuntu 4.4.0-122.146-generic 4.4.117
Uname: Linux 4.4.0-122-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.16
Architecture: amd64
Date: Wed Jun 20 11:52:38 2018
ProcEnviron:
 SHELL=/bin/bash
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 LANGUAGE=en_US:
SourcePackage: docker.io
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.default.docker: 2018-05-01T15:39:38.793397