Bump docker to 2.3.1

Bump docker to 2.3.1

Also changed db-migrator to base off of 2.3.0 jre-alpine version.  (We
have a "chicken and egg" problem where the migrator can't point to the
current docker version, as it hasn't been released yet, thus pointing it
to the latest released version, which is one back.)

Issue-ID: POLICY-3587
Change-Id: Idf34e9c6a724e31f16636388e239bed059c5dd3e
Signed-off-by: Jim Hahn <jrh3@att.com>