[20.10 backport] Enable ssh forwarding when building a remote target

[crazy-max]

fixes #4112

• backport of Enable ssh forwarding when building a remote target #3050

cc @thaJeztah

[crazy-max]

Looks like it needs a vendoring pass: https://ci-next.docker.com/public/blue/organizations/jenkins/cli/detail/PR-4115/1/pipeline#step-22-log-224

#14 [10/11] RUN ./scripts/build/binary
#14 0.580 Building static docker-linux-amd64
#14 1.631 cli/command/image/build_buildkit.go:33:2: cannot find package "github.com/moby/buildkit/util/gitutil" in any of:
#14 1.631 	/go/src/github.com/docker/cli/vendor/github.com/moby/buildkit/util/gitutil (vendor tree)
#14 1.631 	/usr/local/go/src/github.com/moby/buildkit/util/gitutil (from $GOROOT)
#14 1.631 	/go/src/github.com/moby/buildkit/util/gitutil (from $GOPATH)
#14 ERROR: process "/bin/sh -c ./scripts/build/binary" did not complete successfully: exit code: 1

[crazy-max]

Actually it needs moby/buildkit#2056 to be backported to BuildKit v0.8 branch so might not be worth it to backport this change.