Ensure DOCKER_IMAGE validation always runs first

Move DOCKER_IMAGE validation before TiDB check so validation always happens.
This ensures we fail early if DOCKER_IMAGE is missing, regardless of mode.

Author: zph

mysql/testcontainers_helper.go

@@ -163,21 +163,23 @@ func contains(s, substr string) bool {
 // The image parameter is ignored - TestMain uses DOCKER_IMAGE env var
 // This function validates that DOCKER_IMAGE is set and fails early if not
 func getSharedMySQLContainer(t *testing.T, image string) *MySQLTestContainer {
-	// Check if we're in TiDB mode - if so, this function shouldn't be called
-	tidbVersion := os.Getenv("TIDB_VERSION")
-	if tidbVersion != "" {
-		t.Fatalf("ERROR: getSharedMySQLContainer called but TIDB_VERSION is set. " +
-			"TiDB tests should use the shared TiDB cluster from TestMain, not getSharedMySQLContainer.")
-	}
-
 	// Validate that DOCKER_IMAGE is set (required by TestMain for MySQL/Percona/MariaDB)
+	// This validation must always be present - fail early if DOCKER_IMAGE is empty
 	dockerImage := os.Getenv("DOCKER_IMAGE")
 	if dockerImage == "" {
 		t.Fatalf("ERROR: DOCKER_IMAGE environment variable is not set. This is required for MySQL/Percona/MariaDB tests.\n" +
 			"Please set DOCKER_IMAGE to the appropriate Docker image (e.g., mysql:5.6, percona:8.0, mariadb:10.10)\n" +
 			"The 'image' parameter to getSharedMySQLContainer is ignored - use DOCKER_IMAGE env var instead.")
 	}
 
+	// Check if we're in TiDB mode - if so, this function shouldn't be called
+	// TiDB tests use sharedTiDBCluster, not sharedContainer
+	tidbVersion := os.Getenv("TIDB_VERSION")
+	if tidbVersion != "" {
+		t.Fatalf("ERROR: getSharedMySQLContainer called but TIDB_VERSION is set. " +
+			"TiDB tests should use the shared TiDB cluster from TestMain, not getSharedMySQLContainer.")
+	}
+
 	// Validate that the provided image matches DOCKER_IMAGE (if provided)
 	if image != "" && image != dockerImage {
 		t.Fatalf("ERROR: getSharedMySQLContainer called with image '%s' but DOCKER_IMAGE is set to '%s'.\n"+