Ergo Enn

Turns out also `STRAPI_ADMIN_BACKEND_URL` needs to be set during **build time** since some places us it for url generation e.g. https://github.com/strapi/strapi/blob/171c6daa568f68cd001f5c65d22fb934cd295edf/packages/core/admin/admin/src/render.ts#L31C47-L31C71

Hi! While I am not the OP, I think I recreated a minimal repro that explains the problem. https://gist.github.com/ErgEnn/ae8bda788bdc9c4fb5807774510d5d35