swh-loader-svn: Build rsvndump dependency

This creates a new utility image holding the rsvndump pushed in the gitlab registry.

And then adapts the svn loader image to expose that new dependency tool [1]. That will
allow development to use rsvndump instead of svnrdump.

[1]
```
$ docker build -t swh-loader-svn .
$ docker run -ti --rm --user root --entrypoint rsvndump swh-loader-svn --version
rsvndump 0.6.1
Copyright (C) 2008-present Jonas Gehring <jonas@jgehring.net>
Released under the GNU General Public License.
```