Use SkipOutput instead of -1 for output indexes to clarify semantics.

Signed-off-by: Ben Longo <[email protected]>

Author: benlongo

client/llb/exec.go

@@ -339,7 +339,7 @@ func (e *ExecOp) Marshal(ctx context.Context, c *Constraints) (digest.Digest, []
 			inputIndex = pb.Empty
 		}
 
-		outputIndex := pb.OutputIndex(-1)
+		outputIndex := pb.SkipOutput
 		if !m.noOutput && !m.readonly && m.cacheID == "" && !m.tmpfs {
 			outputIndex = pb.OutputIndex(outIndex)
 			outIndex++

client/llb/fileop.go

@@ -702,7 +702,7 @@ func (f *FileOp) Marshal(ctx context.Context, c *Constraints) (digest.Digest, []
 	pop.Inputs = state.inputs
 
 	for i, st := range state.actions {
-		output := pb.OutputIndex(-1)
+		output := pb.SkipOutput
 		if i+1 == len(state.actions) {
 			output = 0
 		}