Change 'Uploading context' wording

This changes the wording in the docker build output from "Uploading context" to the less ambiguous "Sending build context to docker daemon". As discussed in #2342, "Uploading" has negative connotations when the daemon is running locally and what is being sent isn't altogether clear from output alone. Fixes #2342

Docker-DCO-1.1-Signed-off-by: Ryan Stelly ryan.stelly@live.com (github: FLGMwt)