Include build context structure when debugging with DAP

[rcjsuen]

Description

When using DAP, I can navigate the file system through the variables view of my DAP client. It would be nice if we could add a node to show the "build context". Lots of Dockerfiles use a simple COPY . . but people may not realize what's actually in the build context because of the .dockerignore file. Showing the build context when debugging could help demystify this.

[rcjsuen]

This should work for a file system or when building with a tarball as a context. I know other types of build contexts are supported as well but not sure which other ones make sense.