Docker in Docker cannot mount volume

Question

I am running a Jenkins cluster where in the Master and Slave, both are running as a Docker containers.

The Host is latest boot2docker VM running on MacOS.

T

Answer 1

I have same problem in Gitlab CI, I solved this by using docker cp to do something like mount

script:
  - docker run --name ${CONTAINER_NAME} ${API_TEST_IMAGE_NAME}
after_script:
  - docker cp ${CONTAINER_NAME}:/code/newman ./
  - docker rm ${CONTAINER_NAME}

Answer 2

Based from the description mentioned by @ZephyrPLUSPLUS here is how I managed to solve this:

vagrant@vagrant:~$ hostname
vagrant
vagrant@vagrant:~$ ls -l /home/vagrant/dir-new/
total 4
-rw-rw-r-- 1 vagrant vagrant 10 Jun 19 11:24 file-new
vagrant@vagrant:~$ cat /home/vagrant/dir-new/file-new 
something
vagrant@vagrant:~$ docker run --rm -it -v /var/run/docker.sock:/var/run/docker.sock  docker /bin/sh
/ # hostname
3947b1f93e61
/ # ls -l /home/vagrant/dir-new/
ls: /home/vagrant/dir-new/: No such file or directory
/ # docker run -it --rm -v /home/vagrant/dir-new:/magic ubuntu /bin/bash
root@3644bfdac636:/# ls -l /magic
total 4
-rw-rw-r-- 1 1000 1000 10 Jun 19 11:24 file-new
root@3644bfdac636:/# cat /magic/file-new 
something
root@3644bfdac636:/# exit
/ # hostname
3947b1f93e61
/ # vagrant@vagrant:~$ hostname
vagrant
vagrant@vagrant:~$ 

So docker is installed on a Vagrant machine. Lets call it vagrant. The directory you want to mount is in /home/vagrant/dir-new in vagrant. It starts a container, with host 3947b1f93e61. Notice that /home/vagrant/dir-new/ is not mounted for 3947b1f93e61. Next we use the exact location from vagrant, which is /home/vagrant/dir-new as the source of the mount and specify any mount target we want, in this case it is /magic. Also note that /home/vagrant/dir-new does not exist in 3947b1f93e61. This starts another container, 3644bfdac636. Now the contents from /home/vagrant/dir-new in vagrant can be accessed from 3644bfdac636.

I think because docker-in-docker is not a child, but a sibling. and the path you specify must be the parent path and not the sibling's path. So any mount would still refer to the path from vagrant, no matter how deep you do docker-in-docker.

Answer 3

You can solve this passing in an environment variable. Example:

.
├── docker-compose.yml
└── my-volume-dir
    └── test.txt

In docker-compose.yml

version: "3.3"
services:
  test:
    image: "ubuntu:20.04"
    volumes:
      - ${REPO_ROOT-.}/my-volume-dir:/my-volume
    entrypoint: ls /my-volume

To test run

docker run -e REPO_ROOT=${PWD} \
   -v /var/run/docker.sock:/var/run/docker.sock \
   -v ${PWD}:/my-repo \
   -w /my-repo \
   docker/compose \
   docker-compose up test

You should see in the output:

test_1  | test.txt

Answer 4

Regarding your use case related to Jenkins, you can simply fake the path by creating a symlink on the host:

ln -s $HOST_JENKINS_DATA_DIRECTORY/jenkins_data /var/jenkins_home