arm64 image is built with amd64 exporter

[martin-helmich]

This is a follow-up to #101.

The arm64v8 image is built with an amd64 build of the varnish-exporter binary, which is a bad idea for obvious reasons.

Apparently, the upstream project does not release an amd64 build that we could just use out of the box, so we'll need to think of something. I remember we'd already discussed forking and building the exporter by ourselves in #81, so maybe that thought needs to be picked up again.