who: add "Docker Images" link to "Debian" project

As discussed in https://lists.reproducible-builds.org/pipermail/rb-general/2020-January/001791.html, this adds a "Docker Images" link to the https://reproducible-builds.org/who/#Debian. 🤘

I contemplated creating a new "Docker" section as well, but IMO it doesn't feel quite appropriate -- the Docker project build itself is not reproducible (yet? 😄), and I'm not sure it's really tenable to link/highlight every reproducibility effort within the Docker ecosystem (in other words, in the context of "Docker", what makes the Debian Docker image any more notable than any other Docker image or project being reproducible?).

Hopefully I got this in the right place, right syntax, etc, but if not I'm happy to adjust, rebase, amend, etc. 👍