Building our own Redis Cluster image for CI pipeline

In gitlab!120566 (merged), the MR adds Redis-Cluster into the pipeline for the application to run specs against. This gives us a higher degree of confidence when migrating from Redis to Redis Cluster.

The MR uses Docker images from https://hub.docker.com/r/grokzen/redis-cluster but creating our own image would enable the pipeline to be more robust.

See originating issue: gitlab-com/gl-infra/scalability#2327