linux - 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.

To allow Jenkins to be able to perform deployment using Docker, I have mounted the docker.sock and docker client from the host to the Jenkins container like this :-

docker run -v /var/run/docker.sock:/var/run/docker.sock -v $(which docker):/usr/bin/docker -v $HOST_JENKINS_DATA_DIRECTORY/jenkins_data:/var/jenkins_home -v $HOST_SSH_KEYS_DIRECTORY/.ssh/:/var/jenkins_home/.ssh/ -p 8080:8080 jenkins

I am facing issues while mounting a volume to Docker containers that are run inside the Jenkins container. For example, if I need to run another Container inside the Jenkins container, I do the following :-

sudo docker run -v $JENKINS_CONTAINER/deploy.json:/root/deploy.json $CONTAINER_REPO/$CONTAINER_IMAGE 

The above runs the container, but the file "deploy.json" is NOT mounted as a file, but instead as a "Directory". Even if I mount a Directory as a Volume, I am unable to view the files in the resulting container.

Is this a problem, because of file permissions due to Docker in Docker case?

Answer 1

A Docker container in a Docker container uses the parent HOST's Docker daemon and hence, any volumes that are mounted in the "docker-in-docker" case is still referenced from the HOST, and not from the Container.

Therefore, the actual path mounted from the Jenkins container "does not exist" in the HOST. Due to this, a new directory is created in the "docker-in-docker" container that is empty. Same thing applies when a directory is mounted to a new Docker container inside a Container.

Very basic and obvious thing which I missed, but realized as soon I typed the question.