title: How to connect to docker daemon in Bitbucket pipeline
body: <p>I want to run a docker container in a Bitbucket pipeline. But I cannot execute any commands as it claims the docker daemon is not running. However, I did not find any way to start it.</p>
  <pre><code>INFO     Running command 'which docker' in '/opt/atlassian/pipelines/agent/build'
  INFO     STDOUT b'/usr/bin/docker\n'
  INFO     STDERR b''
  INFO     Running command 'docker --version' in '/opt/atlassian/pipelines/agent/build'
  INFO     STDOUT b'Docker version 19.03.15, build 99e3ed8\n'
  INFO     STDERR b''
  INFO     Running command 'systemctl start docker' in '/opt/atlassian/pipelines/agent/build'
  INFO     STDOUT b''
  INFO     STDERR b'/bin/sh: 1: systemctl: not found\n'
  INFO     Running command 'sudo systemctl start docker' in '/opt/atlassian/pipelines/agent/build'
  INFO     STDOUT b''
  INFO     STDERR b'/bin/sh: 1: sudo: not found\n'
  INFO     Trying to remove old Docker container...
  INFO     Running command 'docker container rm test_container' in '/opt/atlassian/pipelines/agent/build'
  INFO     STDOUT b''
  INFO     STDERR b'Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?\n'
  </code></pre>
  <p>I have read that it is common to pipe the outer daemon into the container somehow but I could not find any way how to achieve this.</p>
  <p>So can anyone tell me how this is supposed to work?</p>
  <p>I am executing the commands within a python process with &quot;subprocess.Popen(...)&quot; by the way.</p>
  <p>This is the corresponding step in the bitbucket-pipelines.yml:</p>
  <pre><code>  - step: &amp;test-docker
        name: Build the docker container and run the tests against it
        script:
        - pip install tox
        - docker info
        - tox -e test-docker
        services:
          - docker
  </code></pre>