Fixes #2046: make target stage lookup case insensitive (#2047)

Author: hypnoce

pkg/dockerfile/dockerfile.go

@@ -197,7 +197,7 @@ func targetStage(stages []instructions.Stage, target string) (int, error) {
 		return len(stages) - 1, nil
 	}
 	for i, stage := range stages {
-		if stage.Name == target {
+		if strings.EqualFold(stage.Name, target) {
 			return i, nil
 		}
 	}

pkg/dockerfile/dockerfile_test.go

@@ -261,6 +261,9 @@ func Test_targetStage(t *testing.T) {
 	FROM scratch AS second
 	COPY --from=0 /hi /hi2
 
+	FROM scratch AS UPPER_CASE
+	COPY --from=0 /hi /hi2
+
 	FROM scratch
 	COPY --from=second /hi2 /hi3
 	`
@@ -280,10 +283,16 @@ func Test_targetStage(t *testing.T) {
 			targetIndex: 1,
 			shouldErr:   false,
 		},
+		{
+			name:        "test valid upper case target",
+			target:      "UPPER_CASE",
+			targetIndex: 2,
+			shouldErr:   false,
+		},
 		{
 			name:        "test no target",
 			target:      "",
-			targetIndex: 2,
+			targetIndex: 3,
 			shouldErr:   false,
 		},
 		{