Glauber "eagle eyes" Costa pointed out that the Scylla logo used in our
Docker image documentation looks broken because it's missing the Scylla
text.
Fix the problem by using the Scylla mascot instead.
Message-Id: <1471525154-2800-1-git-send-email-penberg@scylladb.com>
(cherry picked from commit 2bf5e8de6e)