Use the current thread's classloader for classes (#6926)

Switch from using the caller's classloader to using the current thread's
classloader.

Author: snowe2010

core/src/main/java/org/testcontainers/utility/ImageNameSubstitutor.java

@@ -33,7 +33,12 @@ public static synchronized ImageNameSubstitutor instance() {
                 ImageNameSubstitutor configuredInstance;
                 try {
                     configuredInstance =
-                        (ImageNameSubstitutor) Class.forName(configuredClassName).getConstructor().newInstance();
+                        (ImageNameSubstitutor) Thread
+                            .currentThread()
+                            .getContextClassLoader()
+                            .loadClass(configuredClassName)
+                            .getConstructor()
+                            .newInstance();
                 } catch (Exception e) {
                     throw new IllegalArgumentException(
                         "Configured Image Substitutor could not be loaded: " + configuredClassName,