Docker-in-docker setup for dev

[victorb]

Sets up basic docker-in-docker with one example job for demonstrating how it works. Jenkinsfile looks like this:

node {
    stage("Prepare") {
        sh "echo hello > file"
        sh "echo 'FROM busybox\nCOPY file /hello' > Dockerfile"
    }
    stage("Build") {
        sh "docker build -t ipfs/dind-test ."
    }
    stage("Test") {
        sh "docker run ipfs/dind-test cat /hello"
    }
}

Confirmed to work on at least Ubuntu. Didn't get it to work on OSX but since none of us are using it, maybe we should just have linux-like system as requirement for running dev-env...

[ghost]

LGMT :+1:

We need to make double-double-sure that we don't allow non-trusted Jenkinsfiles.