bump VERSION to v1.12.0-rc1

Signed-off-by: Tibor Vass <tibor@docker.com>
diff --git a/VERSION b/VERSION
index 381cf02..1f48802 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.12.0-dev
+1.12.0-rc1