how to install/start docker engine service inside a docker container

Question

I am running jenkins off of the official docker container for jenkins. And i have the following dockerfile following the instructions at https://docs.docker.com/engine/installation/linux/debian/

FROM jenkins:2.32.1

# install docker inside this container
USER root
    # Install Docker inside Jenkins
    RUN apt-get update
    RUN apt-get purge "docker.io*"
    RUN apt-get update
    RUN apt-get install -y apt-transport-https ca-certificates gnupg2
    RUN apt-key adv \
       --keyserver hkp://ha.pool.sks-keyservers.net:80 \
       --recv-keys 58118E89F3A912897C070ADBF76221572C52609D
    RUN echo "deb https://apt.dockerproject.org/repo debian-jessie main" > /etc/apt/sources.list.d/docker.list
    RUN apt-get update
    RUN apt-cache policy docker-engine
    RUN apt-get update
    RUN apt-get install -y docker-engine
    RUN gpasswd -a jenkins docker
    USER jenkins

Then i perform the following:

• build an image from this dockerfile
• run a container from the image
• execute bash in the container as root user
• run sudo docker service start in bash inside the container

Here is what i get:

root@1e0f4b325d58:/# sudo service docker start
mount: permission denied
rmdir: failed to remove ‘cpu’: Read-only file system
mount: permission denied
rmdir: failed to remove ‘cpuacct’: Read-only file system
mount: permission denied
rmdir: failed to remove ‘net_cls’: Read-only file system
mount: permission denied
rmdir: failed to remove ‘net_prio’: Read-only file system
/etc/init.d/docker: 96: ulimit: error setting limit (Operation not permitted)

Answer 1

You need to run the container as a privileged container if you want to run docker inside docker.

So something like this (1) is required:

docker run --privileged your_image:tag

You also need to be careful with iptables and App Armour, but this works after a bit of tinkering.

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The alternative is to give access to the docker daemon inside the container, like so (2):

docker run -v /var/run/docker.sock:/var/run/docker.sock your_image:tag

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Ref:

1 https://blog.docker.com/2013/09/docker-can-now-run-within-docker/

2 https://jpetazzo.github.io/2015/09/03/do-not-use-docker-in-docker-for-ci/