If you have found an official Docker image that is still shellshock vulnerable, the library maintainers [1] would love to hear from you as they take that stuff quite seriously. As far as I know the entire library is fully patched.
As one of the maintainers in question, I'd absolutely mirror this whole statement: if any of the image upstreams have an important update available that isn't applied, we're very interested in rectifying that.
[1] https://github.com/docker-library/official-images