vendor: update buildkit to v0.8

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/moby/buildkit/solver/errdefs/exit.go

@@ -0,0 +1,34 @@
+package errdefs
+
+import "fmt"
+
+const (
+	// ContainerdUnknownExitStatus is returned when containerd is unable to
+	// determine the exit status of a process. This can happen if the process never starts
+	// or if an error was encountered when obtaining the exit status, it is set to 255.
+	//
+	// This const is defined here to prevent importing github.com/containerd/containerd
+	// and corresponds with https://github.com/containerd/containerd/blob/40b22ef0741028917761d8c5d5d29e0d19038836/task.go#L52-L55
+	ContainerdUnknownExitStatus = 255
+)
+
+// ExitError will be returned when the container process exits with a non-zero
+// exit code.
+type ExitError struct {
+	ExitCode uint32
+	Err      error
+}
+
+func (err *ExitError) Error() string {
+	if err.Err != nil {
+		return err.Err.Error()
+	}
+	return fmt.Sprintf("exit code: %d", err.ExitCode)
+}
+
+func (err *ExitError) Unwrap() error {
+	if err.Err == nil {
+		return fmt.Errorf("exit code: %d", err.ExitCode)
+	}
+	return err.Err
+}