Update git submodules

* Update docs/submodules/releng from branch 'master'
  to 0f7b214d1b178bc4146dbd54ccd48f9fc9d9491e
  - Clear former docker images

    Change-Id: Ia69081bf7724fde7dbf54815ac594fd5c38a8f66
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 0934737..0f7b214 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 0934737eb8a9a105cab01784480eb09d09820250
+Subproject commit 0f7b214d1b178bc4146dbd54ccd48f9fc9d9491e