Correct docker url in base.bst

Correct url path in base.bst for the x86_64 image.

Without this we get the following error when we try to track the dependencies:

[00:00:00][????????][track:base.bst  ] FAILURE 404 Client Error: NOT FOUND for url: https://registry.hub.docker.com/v2/buildstream/image-tools//manifests/latest

Notice this url contains '//'