build: use gateway's solve context to allow cancelling getResultAt

Signed-off-by: Justin Chadwell <me@jedevc.com>

build/build.go

@@ -920,7 +920,7 @@ func BuildWithResultHandler(ctx context.Context, nodes []builder.Node, opt map[s
 							}
 							results.Set(resultKey(dp.driverIndex, k), res)
 							if resultHandleFunc != nil {
-								resultCtx, err := NewResultContext(cc, so, res)
+								resultCtx, err := NewResultContext(ctx, cc, so, res)
 								if err == nil {
 									resultHandleFunc(dp.driverIndex, resultCtx)
 								} else {

build/result.go

@@ -19,8 +19,7 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
-func NewResultContext(c *client.Client, solveOpt client.SolveOpt, res *gateway.Result) (*ResultContext, error) {
+func NewResultContext(ctx context.Context, c *client.Client, solveOpt client.SolveOpt, res *gateway.Result) (*ResultContext, error) {
-	ctx := context.Background()
 	def, err := getDefinition(ctx, res)
 	if err != nil {
 		return nil, err