chore(deps): update postgres docker tag to v15 (#2423)

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This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| postgres | major | `12.16-alpine` -> `15.4-alpine` |

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docker-compose.gitpod.yml

@@ -2,7 +2,7 @@ version: '3'
 
 services:
   gitea-database:
-    image: postgres:12.16-alpine
+    image: postgres:15.4-alpine
     environment:
       POSTGRES_USER: gitea
       POSTGRES_PASSWORD: 123456