fix broken build pipeline

* pinned openapi-generator-cli to v6.5.0

Dockerfile

 # This Dockerfile is just used for testing purposes and therefore builds in tox
 # to the image.
 
-FROM openapitools/openapi-generator-cli as builder
+FROM openapitools/openapi-generator-cli:v6.5.0 as builder
 
 WORKDIR /generated