Add Werkzeug to fix build issue

Hey there, I had the same issue as @zurialzurial in prakhar1989#392, gonna suggest just adding this in, although I'll admit I don't know if this is the exact right thing to do more broadly (like if it could cause issues elsewhere).

flask-app/requirements.txt

@@ -1 +1,2 @@
 Flask==2.0.2
+Werkzeug==2.2.2