chore(deps): update elixir docker tag to v1.15.6 (#46)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| elixir | docker | patch | `1.15.4-slim` -> `1.15.6-slim` |
| elixir |  | patch | `1.15.4-slim` -> `1.15.6-slim` |

---

This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).
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Reviewed-on: #46
Co-authored-by: renovate-bot <renovate-bot@dubas.dev>
Co-committed-by: renovate-bot <renovate-bot@dubas.dev>

docker-compose.yml

@@ -13,7 +13,7 @@ services:
       - 'db_data:/var/lib/postgresql/data'
     restart: unless-stopped
   app:
-    image: 'elixir:1.15.4-slim'
+    image: 'elixir:1.15.6-slim'
     hostname: app
     depends_on:
       - db