Mount linux image in docker container

Question

For a project I need to mount a linux image inside a docker container running ubuntu. The image I want to mount is Raspbian. I need to access the linux filesystem of the image and add a file.

I access the image by mounting the folder with the volume flag:

docker run -it -v /path/to/image/folder:/default ubuntu /bin/bash

With fdisk -l raspbian.img I found the offset:

Disk raspbian.img: 1.3 GiB, 1389363200 bytes, 2713600 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x5a7089a1

Device        Boot  Start     End Sectors  Size Id Type
raspbian.img1        8192  137215  129024   63M  c W95 FAT32 (LBA)
raspbian.img2      137216 2713599 2576384  1.2G 83 Linux

Now when I try to mount the image with mount -o loop,offset=$((137216*512)) raspbian.img /mnt/ I get mount: /mnt/: mount failed: Unknown error -1. Can someone explain if I can mount a linux image in a running docker container and if so how?

Edit

Doing the same mount operations in vagrant works perfectly. Are there some limitations to docker mounting filesystems?

Answer 1

Are there some limitations to docker mounting filesystems?

Yes. A standard Docker container has a number of security restrictions in place. As you have discovered, you can't mount new filesystems. You are also unable to modify the network environment of the container.

One solution is simply to perform the mount operation on the host, and then expose the mounted directory into the container using the -v argument to docker run. Something like:

# losetup -fP --show raspbian.img
/dev/loop0
# mount /dev/loop0p2 /mnt
# docker run -v /mnt:/raspbian ubuntu bash

But if you really want to perform the mount inside the container, you can run a privileged container, using the --privileged option to docker run. This removes most of the restrictions normally placed on a Docker container:

• You will have complete access to he host's /dev.
• You will be able to mount filesystems.
• You will be able to modify the network configuration inside the container.

For example:

# docker run -it --rm --privileged -v /images:/images ubuntu bash

Now I can inspect the image:

root@30f80d4598dc:/# fdisk -l /images/2016-09-23-raspbian-jessie-lite.img 
Disk /images/2016-09-23-raspbian-jessie-lite.img: 1.3 GiB, 1389363200 bytes, 2713600 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x5a7089a1

Device                                       Boot  Start     End Sectors  Size Id Type
/images/2016-09-23-raspbian-jessie-lite.img1        8192  137215  129024   63M  c W95 FAT
/images/2016-09-23-raspbian-jessie-lite.img2      137216 2713599 2576384  1.2G 83 Linux

And mount it:

root@952a75f105ee:/# mount -o loop,offset=$((137216*512))  /images/2016-09-23-raspbian-jessie-lite.img /mnt
root@952a75f105ee:/# ls /mnt
bin   dev  home  lib64       media  opt   root  sbin  sys  usr
boot  etc  lib   lost+found  mnt    proc  run   srv   tmp  var
root@952a75f105ee:/#