This is A Saurceful Of Go that between Matrix Media Repo instances and Ingresses.
It has one job: rewrite Host headers. The reason for this is that matrix-media-repo wants Host: hackerspace.pl (MXID domain) while our traffic comes in with Host: matrix.hackerspace.pl (actual Host at which we serve Matrix).
Locally:
$ bazel run //app/matrix/media-repo-proxy -- -downstream_host=matrix.hackerspace.pl -upstream_host=hackerspace.pl -upstream=foo.bar.svc.cluster.local:8080
In prod, should be part of jsonnet infra and be brought up as needed.