anchor.diff:  
   ## Configuring the build step
   
  -Firstly we need to add the following environment variable to our build, either on an individual build step or as a pipeline-level environment variable:
  +To run your steps in a container, we recommend using [plugins](/docs/agent/v3/plugins). If you want to use docker without plugins, see [Using Docker without plugins](/docs/builds/docker-containerized-builds#using-docker-without-plugins)
   
  -```bash
  -BUILDKITE_DOCKER_COMPOSE_CONTAINER=app
  +At the start of your `pipeline.yml`, add a step to build an image with the docker plugin. This example prebuilds the `app` image and uploads it to a registry:
  +```
  +steps:
  +  - name: "Docker Build"
  +    plugins:
  +      docker-compose#v1.3.2:
  +        build: app
  +        image-repository: index.docker.io/org/repo
   ```
   
  -This will tell the agent to run each build step like so:
  -
  -```bash
  -docker-compose -f docker-compose.yml run app <command>
  -```
  -
  -If you need, you can also specify an alternate Docker Compose config file:
  +To run later steps in the image you have built, use the `plugins` attribute on command steps. This example runs a test suite concurrently in the `app` image that was built in the previous example:
   
   ```
  -BUILDKITE_DOCKER_COMPOSE_FILE=docker-compose.buildkite.yml
  +- name: "Docker Test %n"
  +    command: test.sh
  +    parallelism: 25
  +    plugins:
  +      docker-compose#v1.3.2:
  +        run: app
   ```
   
   ## Adding buildkite-agent to the docker group
  
positive.code: # Containerized Builds with Docker
  
  The Buildkite Agent ships with built-in support for running your builds in Docker containers. Running your builds with Docker allows each pipeline to define and document its testing environment, greatly simplifying your build servers, and provides build isolation when [parallelizing your build](parallelizing-builds).
  
  <div class="Docs__note">
  <p>These instructions are for the current Buildkite Agent v2 stable series. For the v3 beta please use the <a href="https://github.com/buildkite-plugins/docker-buildkite-plugin">Docker Buildkite Plugin</a>.</p>
  </div>
  
  <%= toc %>
  
  
negative.code: Another example of a production docker-compose configuration is from the [buildkite-agent Golang source code](https://github.com/buildkite/agent):
  
  ```
  version: '2.1'
  services:
    agent:
      build: .
      working_dir: /go/src/github.com/buildkite/agent
      volumes:
        - ./:/go/src/github.com/buildkite/agent:cached
      environment:
        - BUILDKITE_BUILD_NUMBER
        - BUILDKITE_JOB_ID
  ```
  
  The above example has an additional volume: the `buildkite-agent` binary itself, so the build scripts can use the `buildkite-agent artifact` and `buildkite-agent meta-data` commands. It also passes through environment variables needed for the artifact command.
  
  ## Testing the config
  
  