🐛 Fix default port for penpot-exporter on nginx entrypoint

By default, penpot-frontend will try to reach penpot-export on port 80,
this is wrong, the exporter listens by default to port 6061.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f