Tonis Tiigi eefa8188e1 build: add experimental support for print flag
Print flag can be used to make additional information
requests about the build and print their results.

Currently Dockerfile supports: outline, targets, subrequests.describe

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
..
binarylog.go build: add experimental support for print flag
binarylog_testutil.go vendor: initial vendor
env_config.go build: add experimental support for print flag
method_logger.go build: add experimental support for print flag
sink.go vendor: update buildkit