fix(docker): Double-quote REDIS_VERSION Docker ARG

This is related to fly-apps#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 fly-apps#18, I felt it would be
a good idea to nip it in the bud.

Dockerfile

@@ -1,5 +1,5 @@
 ARG REDIS_VERSION=6.2.6
-FROM redis:${REDIS_VERSION}-alpine
+FROM redis:"${REDIS_VERSION}"-alpine
 
 COPY start-redis-server.sh /usr/bin/start-redis-server.sh