Comment 34 for bug 1711407

simpler reproducer for this using LXD instead of Docker:

1. install/setup LXD
$ sudo apt install lxd
$ sudo lxd init

2. create two containers
$ lxc launch ubuntu:xenial server
$ lxc launch ubuntu:xenial client

3. make the client privileged (so it can mount smb/cifs)
$ lxc config set client security.privileged true
$ lxc stop client
$ lxc start client

4. set up samba server in container
$ lxc exec server apt update
$ lxc exec server apt install cifs-utils samba
$ lxc exec server mkdir /dummy
$ lxc exec server touch /dummy/emptyfile
$ lxc exec server vim /etc/samba/smb.conf

add this section at the end of the container's smb.conf:

[dummy]
  path = /dummy
  browseable = yes
  read only = yes
  guest ok = yes

$ lxc stop server
$ lxc start server

5. note server container's ip
$ lxc list server

e.g.
| server | RUNNING | 10.109.201.68 (eth0)

6. setup client container
$ lxc exec client mkdir /dummy
$ lxc exec apt update
$ lxc exec apt install cifs-utils

7. stop client container
$ lxc stop client

8. full cycle of client container start through stop
$ lxc start client ; sleep 2 ; lxc exec client -- mount -o guest //10.109.201.68/dummy /dummy ; lxc exec client -- ls -l /dummy ; lxc exec client umount /dummy ; lxc stop -f client

9. repeat client cycle; it should hang due to this bug
$ lxc start client ; sleep 2 ; lxc exec client -- mount -o guest //10.109.201.68/dummy /dummy ; lxc exec client -- ls -l /dummy ; lxc exec client umount /dummy ; lxc stop -f client

Note this force-stops the container, instead of normally stopping it; if the container is allowed to exit normally, the problem isn't reproduced (with these steps, at least). Also note the 'sleep 2' is there to give enough time for the client container network to come up, it may need to be increased if your client container takes longer to assign its ip address (you can edit the client container nw config to use a static address if you want, which should be faster).