Feature - Private Docker registries

Description

Currently, Docker images are pulled from the default registry, Docker Hub. This is a public registry (unless you pay) so everyone can view the image being used for grading. This can be undesirable. Therefore, AuTA needs to support private registries so, for example, this GitLab's registry can be used instead.

Priority

High - to be used in the upcoming Operating Systems lab

Definition of done

• - workers send the X-Registry-Config header
• - assignments can contain this information
• - the API can get and set these values