How can I add a volume to an existing Docker conta

Question

I have a Docker container that I've created simply by installing Docker on Ubuntu and doing:

sudo docker run -i -t ubuntu /bin/bash

I immediately started installing Java and some other tools, spend some time with it and stopped the container by

exit

Then I wanted to add a volume and realised that this is not as straightforward as I thought it would be. If I use sudo docker -v /somedir run ... then I end up with a fresh new container, so I'd have install Java and do what I've already done before just to arrive at a container with a mounted volume.

All the documentation about mounting a folder from the host seems to imply that mounting a volume is something that can be done when creating a container. So the only option I have to avoid reconfiguring a new container from scratch is to commit the existing container to a repository and use that as the basis of a new one whilst mounting the volume.

Is this indeed the only way to add a volume to an existing container?

Answer 1

You can commit your existing container (that is create a new image from container’s changes) and then run it with your new mounts.

Example:

$ docker ps  -a
CONTAINER ID        IMAGE                 COMMAND                  CREATED              STATUS                          PORTS               NAMES
    5a8f89adeead        ubuntu:14.04          "/bin/bash"              About a minute ago   Exited (0) About a minute ago                       agitated_newton

$ docker commit 5a8f89adeead newimagename

$ docker run -ti -v "$PWD/dir1":/dir1 -v "$PWD/dir2":/dir2 newimagename /bin/bash

If it's all OK, stop your old container, and use this new one.

That´s it :)

Answer 2

We don't have any way to add volume in running container, but to achieve this objective you may use the below commands:

Copy files/folders between a container and the local filesystem :-

docker cp [OPTIONS] CONTAINER:SRC_PATH DEST_PATH|-

docker cp [OPTIONS] SRC_PATH|- CONTAINER:DEST_PATH

For reference see:

https://docs.docker.com/engine/reference/commandline/cp/

Answer 3

Unfortunately the switch option to mount a volume is only found in the run command.

docker run --help

-v, --volume list Bind mount a volume (default [])

There is a way you can work around this though so you won't have to reinstall the applications you've already set up on your container.

1. Export your container docker container export -o ./myimage.docker mycontainer
2. Import as an image docker import ./myimage.docker myimage
3. Then docker run -i -t -v /somedir --name mycontainer myimage /bin/bash

Answer 4

I've successfully mount /home/<user-name> folder of my host to the /mnt folder of the existing (not running) container. You can do it in the following way:

1. Open configuration file corresponding to the stopped container, which can be found at /var/lib/docker/containers/99d...1fb/config.v2.json (may be config.json for older versions of docker).

2. Find MountPoints section, which was empty in my case: "MountPoints":{}. Next replace the contents with something like this (you can copy proper contents from another container with proper settings):

   "MountPoints":{"/mnt":{"Source":"/home/<user-name>","Destination":"/mnt","RW":true,"Name":"","Driver":"","Type":"bind","Propagation":"rprivate","Spec":{"Type":"bind","Source":"/home/<user-name>","Target":"/mnt"},"SkipMountpointCreation":false}}

3. Restart the docker service: service docker restart

This works for me with Ubuntu 18.04.1 and Docker 18.09.0

Answer 5

Jérôme Petazzoni has a pretty interesting blog post on how to Attach a volume to a container while it is running. This isn't something that's built into Docker out of the box, but possible to accomplish.

As he also points out

This will not work on filesystems which are not based on block devices.

It will only work if /proc/mounts correctly lists the block device node (which, as we saw above, is not necessarily true).

Also, I only tested this on my local environment; I didn’t even try on a cloud instance or anything like that

YMMV