Bump VERSION to v1.12.1-rc1

Signed-off-by: Tibor Vass <tibor@docker.com>
diff --git a/VERSION b/VERSION
index 0eed1a2..e8e560f 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.12.0
+1.12.1-rc1