Provide linux/riscv64 container images

[tdakkota]

Currently, there is no available image of node_exporter for linux/riscv64:

$ docker pull --platform linux/riscv64 quay.io/prometheus/node-exporter:latest
latest: Pulling from prometheus/node-exporter
no matching manifest for linux/riscv64 in the manifest list entries
$ docker pull --platform linux/riscv64 prom/node-exporter:latest
latest: Pulling from prom/node-exporter
no matching manifest for linux/riscv64 in the manifest list entries

It looks like it was not intended, since #2706 was closed without updating Makefile and .dockerignore.