Ignore generated Dockerfiles

Dockerfiles get generated during the build phase for different
versions of OSes and tools.
Prevent them to be checked in by mistake.

Change-Id: Ibebccc8451d939cbc7ddc9b4787a68cc6310f059
diff --git a/.gitignore b/.gitignore
index eafb910..1f9b168 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 id_rsa*
+Dockerfile-*