Add support for local docker volumes

[itsmegb]

This PR adds support for including local docker volumes in the docker compose file.

I've also updated the readme to include an example implementation.

[itsmegb]

I've just got around to fixing this PR, there was a stupid typo in one of the tags that caused the template generation to fail.

[jasonlearst]

Hi @itsmegb,

Thanks for this PR. For my use case I modified your template to add name: {{volume.name}} to ensure that the volume was named as I described. I was wondering if you considered naming the volumes? Would this be a change you would be willing to add to the PR?

{% for volume in docker_volumes %}
    {{ volume.name }}:
        name: {{ volume.name }}
    {% if volume.external is defined %}
        external: {{ volume.external }}
    {% endif %}
{% endfor %}

[ironicbadger]

sorry i let this go stale. needs a rebase now i think.