refactor: improve util.RandString and fix Dockerfile EXPOSE

[Kaustubh1204]

This PR makes two small improvements:

1. util.RandString

   • Uses the global math/rand generator (auto-seeded in Go 1.20+)
     instead of creating a new rand.Source on each call.
   • Adds strings.Builder.Grow(length) to reduce allocations.
   • Adds deterministic unit tests validating length and character set.

2. Dockerfile

   • Changes EXPOSE 9379:9379 to EXPOSE 9379
     to align with Dockerfile syntax expectations.