fix: stop .gitignore from excluding build/.next (site 502/blank after deploy)

.gitignore's ".next/" rule (no leading slash) matched at any depth, so
build/.next was silently excluded from every commit even though build.sh
populates it. The Docker image was built without the compiled server/static
output, so server.js had nothing to serve. Anchor the rule to the repo root
and commit the compiled output that Dokploy actually needs.

build/.next/server/pages-manifest.json

@@ -0,0 +1,4 @@
+{
+  "/404": "pages/404.html",
+  "/500": "pages/500.html"
+}