docker images: copy redis instead of apk add, take 2
Created by: uwedeportivo
we also need redis-check-aof
see https://github.com/sourcegraph/sourcegraph/pull/12994
https://buildkite.com/sourcegraph/sourcegraph/builds?branch=single_server_dockerfile_redis_alpine_2 (i'm gonna put this on sourcegraph.sgdev.org)
actually the faster https://buildkite.com/sourcegraph/sourcegraph/builds/70835