Imports of dockerfile-frontend command

PathSynopsis
flagPackage flag implements command-line flag parsing.
fmtPackage fmt implements formatted I/O with functions analogous to C's printf and scanf.
github.com/moby/buildkit/frontend/dockerfile/builder
github.com/moby/buildkit/frontend/gateway/grpcclient
github.com/moby/buildkit/util/appcontext
github.com/moby/buildkit/util/bklog
github.com/moby/buildkit/util/grpcutil/encoding/protoPackage proto implements a custom encoding for grpc.
github.com/moby/buildkit/util/stack
osPackage os provides a platform-independent interface to operating system functionality.