Dockerfile: support redis

[maximbaz]

This adds php extensions for redis, so that a user can choose to use redis without having to recompile hauk image as well.

[bilde2910]

The intent of the Docker image was to bundle a production-ready Hauk installation together with memcached, so that the user doesn't need to have any dependencies on other containers. Though it does make sense that a Hauk container supports all the plugins and storage options, if the hoster decides to use an external solution instead. Perhaps it would also make sense to at some point remove the bundled memcached service entirely, or offer a separate Docker image tag that excludes memcached.

I'll await merging this because I need to test building the image, and make sure that adding redis via PECL doesn't require additional dependencies to be installed via apt, as if so, I need to add those dependencies to the Dockerfile to ensure that all the dependencies are present before the extension is built.

[maximbaz]

Yeah I realized the purpose of the image, but I wanted to use redis on my own server instead of memcached, and this simple change allowed me to do it 😉 I agree it would be cool to provide a separate tag without bundled memcached, but honestly I don't care too much - this change alone works well 👍 Once you try this out as well, let me know if you spot anything, I will be happy to assist further 😉