Fix/verify dockerfile parser (#433)

* add failing test case for lowercase 'as'

Signed-off-by: Furkan <furkan.turkal@trendyol.com>
Co-authored-by: Batuhan Apaydın <batuhan.apaydin@trendyol.com>
Co-authored-by: Jake Sanders <jsand@google.com>

cmd/cosign/cli/verify_dockerfile.go

@@ -130,7 +130,13 @@ func getImagesFromDockerfile(dockerfile io.Reader) ([]string, error) {
 func getImageFromLine(line string) string {
 	line = strings.TrimPrefix(line, "FROM") // Remove "FROM" prefix
 	line = os.ExpandEnv(line)               // Substitute templated vars
-	line = strings.Split(line, " AS ")[0]   // Remove the "AS" portion of line
 	fields := strings.Fields(line)
+	for i := len(fields) - 1; i > 0; i-- {
+		// Remove the "AS" portion of line
+		if strings.EqualFold(fields[i], "AS") {
+			fields = fields[:i]
+			break
+		}
+	}
 	return fields[len(fields)-1] // The image should be the last portion of the line that remains
 }

cmd/cosign/cli/verify_dockerfile_test.go

@@ -37,6 +37,11 @@ func TestGetImagesFromDockerfile(t *testing.T) {
 			fileContents: `FROM gcr.io/test/image:latest`,
 			expected:     []string{"gcr.io/test/image:latest"},
 		},
+		{
+			name:         "tag with as",
+			fileContents: `FROM gcr.io/test/image:1.16.5 as build`,
+			expected:     []string{"gcr.io/test/image:1.16.5"},
+		},
 		{
 			name:         "digest",
 			fileContents: `FROM gcr.io/test/image@sha256:d131624e6f5d8695e9aea7a0439f7bac0fcc50051282e0c3d4d627cab8845ba5`,