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

Lots of good info in these posts but I find none of them are very clear about which container they are referring to. So let's label the 3 environments:

• host: H
• docker container running on H: D
• docker container running in D: D2

We all know how to mount a folder from H into D: start D with

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

The challenge is: you want path-on-H to be available in D2 as path-on-D2.

But we all got bitten when trying to mount the same path-on-H into D2, because we started D2 with

docker run ... -v : ...

When you share the docker socket on H with D, then running docker commands in D is essentially running them on H. Indeed if you start D2 like this, all works (quite unexpectedly at first, but makes sense when you think about it):

docker run ... -v : ...

The next tricky bit is that for many of us, path-on-H will change depending on who runs it. There are many ways to pass data into D so it knows what to use for path-on-H, but probably the easiest is an environment variable. To make the purpose of such var clearer, I start its name with DIND_. Then from H start D like this:

docker run ... -v : --env DIND_USER_HOME=$HOME \
    --env DIND_SOMETHING=blabla -v /var/run/docker.sock:/var/run/docker.sock ...

and from D start D2 like this:

docker run ... -v $DIND_USER_HOME: ...