Without this configuration, all mount points are reporting the same
utilisation metrics [1]. With the rslave option, all root mounts from
the host are visible in the container, so we can remove the bind mounts
for /proc and /sys.
[1] https://github.com/prometheus/node_exporter#docker
Change-Id: I4087dc81f9d1fa5daa24b9df6daf1f9e1ccd702f
Closes-Bug: #1961438
b210dcd6e2