fix as keyword in FROM instruction

Signed-off-by: Batuhan Apaydın <batuhan.apaydin@trendyol.com>

cmd/cosign/cli/verify_dockerfile.go

@@ -124,6 +124,7 @@ 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.ReplaceAll(line, "as", "AS") // replace lower "as" with "AS"
 	line = strings.Split(line, " AS ")[0]   // Remove the "AS" portion of line
 	fields := strings.Fields(line)
 	return fields[len(fields)-1] // The image should be the last portion of the line that remains

cmd/cosign/cli/verify_dockerfile_test.go

@@ -39,8 +39,8 @@ func TestGetImagesFromDockerfile(t *testing.T) {
 		},
 		{
 			name:         "tag with as",
-			fileContents: `FROM golang:1.16.5 as build`,
-			expected:     []string{"gcr.io/test/image:latest"},
+			fileContents: `FROM gcr.io/test/image:1.16.5 as build`,
+			expected:     []string{"gcr.io/test/image:1.16.5"},
 		},
 		{
 			name:         "digest",