We'll move to "context" once the grpc, protobuf and protoc-gen-go packages have
done so (see https://github.com/grpc/grpc-go/issues/711)

This also forced me to manually update tonistiigi/fifo to use the old
context path for now.

This change ensures we stay compatible with go1.6.x

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>
(cherry picked from commit fa71c585c9d437605b78a4ae3cd66284d7053ea6)