oci: make mounting oci socket optional

The source path changed in v0.13 and there are reports
that new path can cause error on starting a container. While
this is investigated, check for missing path and make
mounting optional like it was in v0.12.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
(cherry picked from commit 33b347a)

executor/oci/spec.go

@@ -2,6 +2,7 @@ package oci
 
 import (
 	"context"
+	"os"
 	"path"
 	"path/filepath"
 	"runtime"
@@ -197,8 +198,11 @@ func GenerateSpec(ctx context.Context, meta executor.Meta, mounts []executor.Mou
 	}
 
 	if tracingSocket != "" {
-		if mount := getTracingSocketMount(tracingSocket); mount != nil {
-			s.Mounts = append(s.Mounts, *mount)
+		// moby/buildkit#4764
+		if _, err := os.Stat(tracingSocket); err == nil {
+			if mount := getTracingSocketMount(tracingSocket); mount != nil {
+				s.Mounts = append(s.Mounts, *mount)
+			}
 		}
 	}