Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Merge "[arm-build] prune-docker-images on all builders"
  - [arm-build] prune-docker-images on all builders

    Prune dangling references on all arm-build* hosts.
    Remove arm-build2 as it will be retired.

    Change-Id: Ib425b3ef8021fa74d5942609093234cad35c0644
    Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index bc1fad5..705dc15 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit bc1fad5ce90423b34d2fa7020686e4c8c797c48b
+Subproject commit 705dc15a141f84f4a746cb4699b59e8adde3119e