Testcontainers images should not default to DockerHub

[Bhuff1]

Description
ZMSTestUtils uses Testcontainers which defaults to pulling images from Docker Hub. This causes issues in enterprise CI/CD environments that:

1. Cannot access Docker Hub directly (firewall restrictions)
2. Require pulling images from internal registries

Affected images
MySQL container - Used for database tests
Ryuk container - Testcontainers' resource reaper for cleanup

Proposed Solution
Require explicit configuration via environment variables

• ZMS_TEST_MYSQL_IMAGE
• TESTCONTAINERS_RYUK_CONTAINER_IMAGE

Update ZMSTestUtils

private static final String MYSQL_IMAGE_ENV_VAR = "ZMS_TEST_MYSQL_IMAGE";
private static final String DEFAULT_MYSQL_IMAGE = "mysql/mysql-server:8.0";

public static String getMySQLImage() {
    String image = System.getenv("ZMS_TEST_MYSQL_IMAGE");
    if (image == null || image.isEmpty()) {
        image = DEFAULT_MYSQL_IMAGE; // fallback to default if no image specified
    }
    return image;
}

public static MySQLContainer startMemoryMySQL(final String userName, final String password) {
   final String mysqlImage = getMySQLImage();
   ...
}

[Bhuff1]

I'm happy to take care of this.