Bump VERSION to v1.12.2-rc3

Signed-off-by: Victor Vieux <vieux@docker.com>
diff --git a/VERSION b/VERSION
index 93680b4..f0ca823 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.12.2-rc2
+1.12.2-rc3