Cannot specify registry credentials for build step

[HeavyWombat]

I think we have a gap in our setup for the following use case: You have a build that requires a specific container image (for example in the Dockerfile FROM statement), which is hosted in a private repository. We have no means to configure a secret with credentials to be used. If you happen to have configured the output image in the same registry, you get lucky as the container registry credentials for the output image are also used for pulling images. However, if they are different, it will not work. This could be especially interesting in case you hit the DockerHub rate limiting and want to have an authenticated pull from the registry.