respect dockerignore

[samouri]

As @sirreal discovered, .dockerignore files are currently ignored by the tar stream process in the builder. We should respect .dockerignore files when building the build context

[sirreal]

As you noted, this is likely related to the way Docker engine receives a tar archive and Docker build from a tar uses it as the context, it doesn't create a context from the tar. This effectively circumvents the .dockerignore.

At least that's the best guess 🙂