llb: add const for root mount

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

client/llb/state.go

@@ -3,6 +3,7 @@ package llb
 import (
 	_ "crypto/sha256"
 
+	"github.com/moby/buildkit/solver/pb"
 	digest "github.com/opencontainers/go-digest"
 	"github.com/pkg/errors"
 )
@@ -48,7 +49,7 @@ func (s *State) Run(opts ...RunOption) *ExecState {
 		meta:   meta,
 		mounts: []*mount{},
 		root: &mount{
-			dest:      "/",
+			dest:      pb.RootMount,
 			source:    s.source,
 			parent:    s.mount,
 			hasOutput: true,

solver/exec.go

@@ -85,7 +85,7 @@ func (e *execOp) Run(ctx context.Context, inputs []Reference) ([]Reference, erro
 			outputs = append(outputs, active)
 			mountable = active
 		}
-		if m.Dest == "/" {
+		if m.Dest == pb.RootMount {
 			root = mountable
 		} else {
 			mounts = append(mounts, worker.Mount{Src: mountable, Dest: m.Dest})

solver/pb/const.go

@@ -0,0 +1,3 @@
+package pb
+
+const RootMount = "/"