There are two ways of creating custom tags - automatic way using API and manual option adding tags using UI.
When users create a build trigger for Github push, all images built will be tagged with the name of the branch or tag whose change invoked the trigger. Users can add additional tags to built images by clicking on the cogwheel inside the Tags view in Quay and clicking on "Add new tag". This will clone the current image and create a new one with the new tag which will be linked to the master tag (the link is symbolized by a blue connection). Whenever users rebuild the original image, the connected tag will be rebuilt as well.