Fix reading context and dockerfile from stdin with BuildKit
Fix reading context and dockerfile from stdin with BuildKit

Signed-off-by: Alexey Igrychev <2bd2dfe22ab2a7aa2644fa00990f84008652e736@flant.com>

docker/cli,cpuguy83/cli,docker/cli,thaJeztah/cli,cpuguy83/cli,thaJeztah/cli