Add image build log to the image for documentation purpose

@hpvb @tinmanjuggernaut

Do you know of an easy/idiomatic way to include the build logs from the image generation as a file in the image itself? (Like `/root/image_build.log`.)

We currently log the last builds to the `logs` folder in this repo, but they're not tied to the built image and will be replaced by subsequent builds.

Having those logs directly in the image would be useful to review what options were used in the build and whether any issues went unnoticed (like #44).