Use registry:3 image for the integration tests

v3 was recently released which broke the integration tests. Update them
to use the correct config file.

Set the major version to prevent this from happening when v4 is
released.

test/integration/docker/registry/boot.sh

@@ -1,3 +1,3 @@
 #!/bin/sh
 
-exec /entrypoint.sh /etc/docker/registry/config.yml
+exec /entrypoint.sh /etc/distribution/config.yml