Docker and fig support

This adds Docker and Fig support to the bootstrap.sh

## Docker support

By setting the `BUILDBOX_DOCKER` environment variable in your build pipeline, this will automatically build and run your build scripts in a docker container, and clean up afterwards.

For example, say you had a `Dockerfile`:

```
FROM ubuntu
ADD . /app
WORKDIR /app
```

and a `script/tests` file.

Creating a build step for `./script/tests` with env variable `BUILDBOX_DOCKER=true` would cause the agent to run the `script/tests` script in a one-off Docker container.

## Fig support

By setting the `BUILDBOX_FIG_CONTAINER` environment variable in your build pipeline, this will automatically build and run your build scripts using Fig, and clean up afterwards.

For example, say you had a `fig.yml`:

```
app:
  build: .
  links:
    - db:db
db:
  image: postgres
```

and a `Dockerfile`:

```
FROM ubuntu
RUN apt-get update -qq
RUN apt-get install -y postgresql-client
ADD . /app
WORKDIR /app
```

and a `script/db_tests` file.

Creating a build step for `./script/db_tests` with env variable `BUILDBOX_FIG_CONTAINER=app` would cause the agent to run the `script/db_tests` script in a one-off "app" Fig container, with the db service linked and available to that container.