ctr: improve error relative shim path error msg

addresses https://github.com/containerd/containerd/issues/6464

Return an error if a runtime provided is relative.

Add context to the usage for `ctr run --runtime` indicating that
absolute path to runtime binary must be provided.

Signed-off-by: Gavin Inglis <giinglis@amazon.com>

runtime/v2/manager.go

@@ -22,6 +22,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strings"
 	"sync"
 
 	"github.com/containerd/containerd/containers"
@@ -244,6 +245,11 @@ func (m *ShimManager) resolveRuntimePath(runtime string) (string, error) {
 		return runtime, nil
 	}
 
+	// Check if relative path to runtime binary provided
+	if strings.Contains(runtime, "/") {
+		return "", fmt.Errorf("invalid runtime name %s, correct runtime name should be either format like `io.containerd.runc.v1` or a full path to the binary", runtime)
+	}
+
 	// Preserve existing logic and resolve runtime path from runtime name.
 
 	name := shimbinary.BinaryName(runtime)