Reproducer:

$ curl https://mbs.stg.fedoraproject.org/module-build-service/1/module-builds/
{
"items": [
{
"id": 2,
"state": 1
}
],
"meta": {
"first": "http://localhost:10063/module-build-service/1/module-builds/?per_page=10&page=1",
"last": "http://localhost:10063/module-build-service/1/module-builds/?per_page=10&page=1",
"page": 1,
"pages": 1,
"per_page": 10,
"total": 1
}
}

We should return http://mbs.stg.fedoraproject.org/ there. This could need some changes in reverse proxy configuration too, if we for example decide to pass the hostname using the http header...