Add support for private Docker registries in Dockerfile

[R0Wi]

Is your feature request related to a problem? Please describe

Thanks to https://github.com/gitpod-io/gitpod/pull/8550, we're already able to use private Docker registries to pull images from in our .gitpod.yml:

image: my.private-registry/my/image:latest

Unfortunately, like stated in the caveats of the current implementation, there's no support for using private registries in Dockerfiles:

with this implementation, private images can only be used directly, not as part of a Dockerfile (i.e. FROM my-private-repo.com/secret-sauce:latest)

Since dev environments are often built with custom Dockerfiles, it would be great to have support for this, too:

.gitpod.Dockerfile

FROM my.private-registry/my/image:latest
RUN apt-get install mypackage
# ...

.gitpod.yml

image:
    file: .gitpod.Dockerfile

Describe the behaviour you'd like

Add support for using private registries in Dockerfiles which are used to built the gitpod container.

Describe alternatives you've considered

Current alternative would be to prebuild the gitpod image and push it to a private registry. This can be quite cumbersome especially when trying new things.

Additional context

If interested in this feature, I'd really like to participate in any way!