Make Docker build public/automated and build in Docker

[javabrett]

https://hub.docker.com/r/messagebird/pushprom/ is not currently a Public/Automated build (the repo is of course public). Public/Automated builds are superior since a) they are updated and published more regularly and b) they are more easily trusted, since it is instantly clear exactly how the image was built and it was built by an automated system.

I don't think it is simple to change a repo to automated - Docker Hub support might be able to do that, or it may involve deleting/recreating or even renaming the image slug.

The steps for this are:

• Make pushprom build in-Docker
• Modify the Docker Hub repo to make it an automated build