Docker in Docker

I got beef with all y’alls articles

Oh look, another docker-in-docker post. How original. Ok, I get it, there are many of these out there. Here’s my beef with all the ones I could find at 1am last night: when people talk about docker-in-docker, they’re actually talking about docker-as-a-service [2] [3] [4] [5] [7] [8] [9] (or, the worse “just mount docker from your own filesystem” [1] [4] [6] [7]).

That is: docker-in-docker is when you run a container, log into it, and are able to run docker. Docker-as-a-service is when you run a container with dockerd and then linking other containers into that container, which themselves run docker.

Ok, yeah, I made up those arbitrary distinctions. But, this is my blog, so I can do what I want! :)

The big difference is that I can do both on my local laptop, but the latter requires extraorinarily more effort to actually stick in a cloud since most automated “run a container in the cloud” services have no idea wtf you’re talking about trying to link containers. We’re just here to run your one, single container dude.

Ok how do you actually do this

Easy solution:

$ docker run --privileged -it docker:dind /bin/sh
/ # dockerd-entrypoint.sh &
[...]
/ # docker ps
error during connect: Get http://docker:2375/v1.40/containers/json: dial tcp: lookup docker on 192.168.65.1:53: no such host
/ # # womp womp
/ # unset DOCKER_HOST
/ # docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
/ # # huzzah!

So, yeah, unset DOCKER_HOST. What’s happening here is that DOCKER_HOST is being set to tcp://docker:2375, but we want to unset that to make it use the unix socket. See more explanation at https://github.com/docker-library/docker/issues/200.

Petty citations

1: https://docs.gocd.org/current/gocd_on_kubernetes/docker_workflows.html#docker-in-docker-dind

2: https://hub.docker.com/_/docker

3: https://docs.gitlab.com/ee/ci/docker/using_docker_build.html

4: https://itnext.io/docker-in-docker-521958d34efd

5: https://sreeninet.wordpress.com/2016/12/23/docker-in-docker-and-play-with-docker/

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

7: http://blog.teracy.com/2017/09/11/how-to-use-docker-in-docker-dind-and-docker-outside-of-docker-dood-for-local-ci-testing/

8: https://stackoverflow.com/questions/45928958/how-to-run-docker-container-inside-docker-dind

9: https://discourse.drone.io/t/build-and-test-docker-images-with-dind/1808