Added redis data volume to compose setup (#21906)

ref
https://linear.app/ghost/issue/ENG-1956/redis-creates-new-anonymous-volume-each-time-it-boots

- Before this commit, the redis service in our docker compose setup
would create a new anonymous volume each time it boots, with a
non-descriptive, hash-based name. Over time these volumes accumulate and
become a pain to clean up, and it's not immediately obvious what they
are used for.
- This commit adds a persistent data volume for redis data, so it will
reuse the same volume, with a more descriptive title each time it boots.
This eliminates the annoying anonymous volumes, and also gives us data
persistence across boots for the redis service (which can be easily
cleared by deleting the volume).

.devcontainer/compose.yml

@@ -35,3 +35,4 @@ services:
 
 volumes:
   mysql-data:
+  redis-data:

compose.yml

@@ -26,6 +26,8 @@ services:
     ports:
       - "6379:6379"
     restart: always
+    volumes:
+      - redis-data:/data
     healthcheck:
       test: [ "CMD", "redis-cli", "--raw", "incr", "ping" ]
       interval: 1s
@@ -61,3 +63,4 @@ services:
       - "9091:9091"
 volumes:
   mysql-data:
+  redis-data: