[fix] devcontainers: missing Valkey package (#4997)

Use specific branch as `valkey-server` package is only available in trixie and bookworm backports.

Reported https://github.com/searxng/searxng/discussions/4995
Closes https://github.com/searxng/searxng/issues/4996

.devcontainer/devcontainer.json

@@ -1,5 +1,8 @@
 {
   "build": {
+    "args": {
+      "DEBIAN_CODENAME": "bookworm",
+    },
     "dockerfile": "Dockerfile"
   },
   "features": {