fix(docker): Double-quote `REDIS_VERSION` Docker ARG

[shymega]

This is related to https://github.com/fly-apps/redis/pull/18, in the context of resolving double-quote escape issues.

There's no known report of this Dockerfile being an issue in the wild, but given https://github.com/fly-apps/redis/pull/18, I felt it would be a good idea to nip it in the bud.