Add way to specify empty context

[cpuguy83]

Description

Normally if I want effectively an empty context I just do something like docker build -< path/to/file.
However, this does not work with buildx's debugging commands since it requires stdin to be available for interaction.

It would be nice if you can signal to buildx to not send a default context.

[jsternberg]

Can you expand on why you need this? I believe the context won't be transferred if it's not used within the Dockerfile. if the Dockerfile needs to use the context then it'll transfer it.

Another potential way to do this is to use named contexts. You can add something like this to your Dockerfile.

FROM scratch AS mycontext

Then you can use --build-context mycontext=path and it will add the context. If you don't include it, it will use an empty context since there's a stage named scratch. Is that something that would work? I think it might be quite difficult to add this feature to buildx just because it would probably break existing functionality or would be hacky.

[cpuguy83]

Because I don't want to create/manage a dir just to pass to buildx.
The issue is the buildx CLI here:

https://github.com/project-dalec/dalec-vscode-extension/blob/0f7b05fb151e782db37eca5fcb1f5db4062ea1d8/src/commands/runBuildCurrentSpecCommand/helpers/contextHelpers.ts#L8

https://github.com/project-dalec/dalec-vscode-extension/blob/0f7b05fb151e782db37eca5fcb1f5db4062ea1d8/src/commands/runBuildCurrentSpecCommand/runBuildCommand.ts#L269