bootstrap: Rename Dockerfile to Containerfile

As we are using podman to build and publish[1] the bootstrap and golang
image, it makes more sense for these images to use a Containerfile

[1] https://github.com/kubevirt/project-infra/blob/e0f89d2a2ae41deed759fef548c8f4e73362103d/images/publish_multiarch_image.sh#L93C1-L101C9

Signed-off-by: Brian Carey <bcarey@redhat.com>

images/publish_multiarch_image.sh

@@ -67,7 +67,7 @@ EOF
 }
 
 get_base_image() {
-    name="$(cat Dockerfile |grep FROM|awk '{print $2}')"
+    name="$(cat Containerfile |grep FROM|awk '{print $2}')"
     echo "${name}"
 }