Can not unmount a device using "umount" in Docker - build

Question

I don't know why but, umount is not working in docker.
umount: loop3/: must be superuser to umount
Let me share one more thing that is
It creates loop3 under /mnt/loop3 in real machine. Which is most unexpected thing for me, because promises pure virtual environment.
Why? Any solution?
Scenario :
I created docker ubuntu:13.04 to create cross compilation environment.
Docker Linux machine (ubuntu):
Linux 626089eadfeb 3.10.45-1-lts #1 SMP Fri Jun 27 06:44:23 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
Linux Machine (rch Linux):
Linux localhost 3.10.45-1-lts #1 SMP Fri Jun 27 06:44:23 UTC 2014 x86_64 GNU/Linux
Docker Info
Client version: 1.0.1
Client API version: 1.12
Go version (client): go1.3
Git commit (client): 990021a
Server version: 1.0.1
Server API version: 1.12
Go version (server): go1.3
Git commit (server): 990021a

Answer 1

I found the solution :
In default docker run it's not a real Operating system as we expect. It doesn't have permissions to access the devices. So we have to use --privileged While running a docker.
By default, Docker containers are "unprivileged" and cannot, for example, run a Docker daemon inside a Docker container. This is because by default a container is not allowed to access any devices, but a "privileged" container is given access to all devices.
When the operator executes docker run --privileged, Docker will enable to access to all devices on the host as well as set some configuration in AppArmor to allow the container nearly all the same access to the host as processes running outside containers on the host.

Answer 2

In my case it was the mount command of linux but it is also valid for unmount.
The previous solution is valid but my scenario could not execute the command 'docker run' since it was being used in real time.
The command I wanted to mount :
mount -o remount,size=5G /dev/shm
Solution
Verification in the docker :
[docker]$ df -h
Filesystem Size Used Avail Use% Mounted on
shm 64M 4.0K 64M 1% /dev/shm
[docker]$ exit
We look for the ID of our container :
$ docker ps
CONTAINER ID IMAGE
<container_id> nameimage
Let's memorize that beginning of the ID
All the following commands must be executed with sudo (use in the shortest possible time) :
# sudo -i
We go to the folder that contains the docker :
# By default
cd /var/lib/docker/containers/
And we open the folder that begins on .
cd <container_id>
We execute our command with the necessary parameters :
mount -o remount,size=5G shm
(Note: I do not remember if the command must be executed to show the file system)
Finally we enter the Docker and verify that the values ​​have been updated correctly:
[docker]$ df -h
Filesystem Size Used Avail Use% Mounted on
shm 5.0G 0 5.0G 0% /dev/shm
Sources
https://github.com/docker/cli/issues/1278

Answer 3

In my case it was a problem during starting docker service.
A cause was not having enough space left on the hard drive.
After cleaning up the disk and rebooting - docker service started successfully.

Answer 4

Please don't follow the accepted answer --privileged give too much permission consider using --cap-add=CAP_SYS_ADMIN that guard many many function but less than privileged:
Perform a range of system administration operations
including: quotactl(2), mount(2), umount(2),
Here we want the mount(2)/umount(2).
If you want to be safe you can cap-add and in the entrypoint drop this capability with prtcl (it depend on the language you use for C/python it is quite easy, maybe prctl has binding for your language).