Add hack to preserve Dockerfile RUN cache compatibility after mount cache bugfix

Signed-off-by: Aaron Lehmann <alehmann@netflix.com>

solver/llbsolver/ops/exec.go

@@ -97,6 +97,22 @@ func (e *execOp) CacheMap(ctx context.Context, g session.Group, index int) (*sol
 		}
 	}
 
+	// Special case for cache compatibility with buggy versions that wrongly
+	// excluded Exec.Mounts: for the default case of one root mount (i.e. RUN
+	// inside a Dockerfile), do not include the mount when generating the cache
+	// map.
+	if len(op.Mounts) == 1 &&
+		op.Mounts[0].Dest == "/" &&
+		op.Mounts[0].Selector == "" &&
+		!op.Mounts[0].Readonly &&
+		op.Mounts[0].MountType == pb.MountType_BIND &&
+		op.Mounts[0].CacheOpt == nil &&
+		op.Mounts[0].SSHOpt == nil &&
+		op.Mounts[0].SecretOpt == nil &&
+		op.Mounts[0].ResultID == "" {
+		op.Mounts = nil
+	}
+
 	dt, err := json.Marshal(struct {
 		Type    string
 		Exec    *pb.ExecOp