Update git submodules

Update git submodules

* Update docs/submodules/sdc/sdc-docker-base.git from branch 'master'
  to 6cdcb857261019e1dbd154fe6f27a68b124cfb4a
  - Fix build failure of base images

    Upgrade of Alpine linux version of the openjdk:8-jdk-alpine
    base image obsoleted usage of edge repositories.
    These repositories were causing build failures.
    Elasticsearch newly requires an installation of linux-headers package.
    This package is required to build dependencies for plugin berkshelf.

    Issue-ID: SDC-2581
    Signed-off-by: Denis Kasanic <denis.kasanic@tieto.com>
    Change-Id: Ib71846d4d449f035cf7633c8ac786553364efef7