Dockerfile: Install ngrep in swh/base image

That allows to check for statsd information (without having to install it each time we plug to that node)

For example:

docker-compose exec swh-storage bash -c 'ngrep -d lo udp and port 8125'

Related swh-storage!158 (closed)

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Migrated from D1312 (view on Phabricator)