Bump version to 1.10.3-rc1

Signed-off-by: Tibor Vass <tibor@docker.com>

VERSION

diff --git a/VERSION b/VERSION
index 587c5f0..3da9a43 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.10.3
+1.10.3-rc1