replace docker/pkg/signal with github.com/moby/sys/signal

The github.com/docker/docker/pkg/signal package was moved to a separate
module in moby/sys.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/container/start.go

@@ -9,7 +9,7 @@ import (
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
 	"github.com/docker/docker/api/types"
-	"github.com/docker/docker/pkg/signal"
+	"github.com/moby/sys/signal"
 	"github.com/moby/term"
 	"github.com/pkg/errors"
 	"github.com/spf13/cobra"