Update git submodules

* Update docs/submodules/releng from branch 'master'
  to 21865851bb0e798c0ba70e5f33f5e455967c02fa
  - Leverage the right alpine versions per branch

    Change-Id: I27cd46d873e2e48072f6aa23e1d4c5be8e9d1ccf
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 4b81bb7..2186585 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 4b81bb7b15f60b3725c9295aabccb05ab0538e2c
+Subproject commit 21865851bb0e798c0ba70e5f33f5e455967c02fa