Docker warning: Pin redis base image version to a release tag

[katsel]

While working on #179, I ran hadolint against all Dockerfiles.
Not much came up, and this warning looks like it's an easy fix against unpleasant surprises.

$ hadolint docker/redis/Dockerfile 
redis/Dockerfile:1 DL3007 warning: Using latest is prone to errors if the image will ever update. Pin the version explicitly to a release tag

[indy-independence]

I had a quick look at https://hub.docker.com/_/redis?tab=tags&page=1&ordering=last_updated and since I don't think we have any special requirements really we could probably go with "redis:6" or "redis:6.2"