Docs: comment on availability of built images in other build steps - #665

This is already the case when using load: true but yeah you can use the docker driver so you don't need to set load: true.

@crazy-max Just load: true doesn't allow you to use the built image in the next build step with the container driver. This is the point.

@crazy-max I still think it would be useful to have these comments in the “export to Docker” section. It could save a lot of time to others.

@baibaratsky Thanks for the changes. I think in a follow-up we might create a new page to reuse build result as it's a bit out of scope with "Export image to Docker" page. But in the meantime that sounds good to me.

Pushed extra commit with a dedicated page about named contexts. Removed the bake example that will be added in bake-action repo instead.