Added Certbot image

[Megajin]

The Certbot image was missing in the .yml file.
I added the image and set the volumes.

[mjstealey]

@Megajin - I had left the certbot definition outside of the docker-compose scope due to the infrequency of it needing to be run. The init step is once per installation and the renew script is periodic. I'll tend to run the renew script as a cron task every 15 days, so haven't found it to be necessary to wire it into the compose definition as an additional service.

The docker run call for certbot is defined in the init and renew scripts (i.e. from init: https://github.com/mjstealey/wordpress-nginx-docker/blob/master/letsencrypt/letsencrypt-init.sh#L89-L95). It uses the .env file and does some manipulation of the deployed Nginx container to accommodate the well-known directory structure that Let's Encrypt is looking for. Even if certbot is deployed as a service there is still some configuration that needs to be done to have all things work as expected. So I hadn't personally seen the advantage of wiring it as a compose service...

All that said there is no harm in having it defined in compose, but it should be documented as to how it should be used within the context of both init and renew. Perhaps even commented out with some deployment notes as an optional method of use... will have to think on it a bit.

[Megajin]

@mjstealey you are right it is not needed in the setup. I had another problem and was thinking the cause might be the missing image. However I was wrong. So feel free to reject this pull request if you come to the conclusion that it is not needed in the setup.

I used this reference: https://medium.com/@pentacent/nginx-and-lets-encrypt-with-docker-in-less-than-5-minutes-b4b8a60d3a71