add: Redis Extension Support

[berksmbl]

I know it is not required for standard installation, but in most cases it can be difficult for the uninitiated to install. It would be very helpful if it came pre-installed.

[tianon]

See https://hub.docker.com/_/wordpress, especially https://github.com/docker-library/docs/tree/master/wordpress#adding-additional-libraries--extensions:

Adding additional libraries / extensions

This image does not provide any additional PHP extensions or other libraries, even if they are required by popular plugins (e.g. it cannot send e-mails). There are an infinite number of possible plugins, and they potentially require any extension PHP supports. Including every PHP extension that exists would dramatically increase the image size.

If you need additional PHP extensions, you'll need to create your own image FROM this one. The documentation of the php image explains how to compile additional extensions. Additionally, an older Dockerfile for wordpress has a simplified example of doing this and a newer version of that same Dockerfile has a much more thorough example.