Is it ok to run docker from inside docker?

Question

I'm running Jenkins inside a Docker container. I wonder if it's ok for the Jenkins container to also be a Docker host? What I'm thinking about is to start a new docker container for each integration test build from inside Jenkins (to start databases, message brokers etc). The containers should thus be shutdown after the integration tests are completed. Is there a reason to avoid running docker containers from inside another docker container in this way?

Answer 1

Running Docker inside Docker (a.k.a. dind), while possible, should be avoided, if at all possible. (Source provided below.) Instead, you want to set up a way for your main container to produce and communicate with sibling containers.

Jérôme Petazzoni — the author of the feature that made it possible for Docker to run inside a Docker container — actually wrote a blog post saying not to do it. The use case he describes matches the OP's exact use case of a CI Docker container that needs to run jobs inside other Docker containers.

Petazzoni lists two reasons why dind is troublesome:

1. It does not cooperate well with Linux Security Modules (LSM).
2. It creates a mismatch in file systems that creates problems for the containers created inside parent containers.

From that blog post, he describes the following alternative,

[The] simplest way is to just expose the Docker socket to your CI container, by bind-mounting it with the -v flag.

Simply put, when you start your CI container (Jenkins or other), instead of hacking something together with Docker-in-Docker, start it with:

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

Now this container will have access to the Docker socket, and will therefore be able to start containers. Except that instead of starting "child" containers, it will start "sibling" containers.

Answer 2

I answered a similar question before on how to run a Docker container inside Docker.

To run docker inside docker is definitely possible. The main thing is that you run the outer container with extra privileges (starting with --privileged=true) and then install docker in that container.

Check this blog post for more info: Docker-in-Docker.

One potential use case for this is described in this entry. The blog describes how to build docker containers within a Jenkins docker container.

However, Docker inside Docker it is not the recommended approach to solve this type of problems. Instead, the recommended approach is to create "sibling" containers as described in this post

So, running Docker inside Docker was by many considered as a good type of solution for this type of problems. Now, the trend is to use "sibling" containers instead. See the answer by @predmijat on this page for more info.