Docker Sign an Image with an existing Certificate With respect to docker's security system.

Is it possible to somehow get images signed by some entity that has trust from a company's CA-cert and then be able to find out who signed the image?

It seems like Docker Trusted Registry (DTR), and Notary in general, have a binary mode of trust: either an image is signed or it is not, but it is not qualified by who performed the signing.
This documentation indicates that it is possible to add collaborators by trusting their key and that it is also possible to revoke access.

Having said that, the answer to:


  either an image is signed or it is not, but it is not qualified by who
  performed the signing


that it is possible to check who signed the image if every collaborator creates their own key and each key is trusted individually.

As notary and dtr were already mentioned, an alternative is Portus. Portus is able to restrict access to docker images, e.g. some users are only allowed to view images, while others may pull them. It is possible to revoke access as well.