Mirror process-exporter image to be resilient to docker registry failure

As discussed with @skarbek in the context of gitlab-com/gl-infra/k8s-workloads/tanka-deployments!371 (merged), we currently are relying on mirror.gcr.io caching for some resilience to docker.io registry failure.

This covers many of the images we use, but not all.

One image that we'd like to have a mirror of is ncabatoff/process-exporter. We currently rely on dev store production images. For consistency, this may be where we want to mirror this one as well.

@gitlab-org/distribution Do we have some sort of process for mirroring images we rely on, or do we generally reference the main docker registry directly?