feat(redis): add Redis 8 Alpine compose file

- Add docker-compose.yaml with password authentication commented out by
  default (no password required)
- Add .env.example with REDIS_PASSWORD documented and commented out

compose/redis/.env.example

@@ -0,0 +1,9 @@
+# Redis (Alpine) — environment variables
+# Copy this file to .env and adjust values for your environment.
+
+# Password for client connections. By default Redis starts with no password
+# (any client can connect). To require authentication:
+#   1. Set REDIS_PASSWORD below
+#   2. Uncomment the "command" line in docker-compose.yaml
+# Clients authenticate with: AUTH <password>
+# REDIS_PASSWORD=redis