Fix image tag command in test (#6301)

`asCanonicalNameString` includes name and tag, so the result contains the tag twice.

Author: SoMuchForSubtlety

core/src/test/java/org/testcontainers/DockerRegistryContainer.java

@@ -100,7 +100,7 @@ public DockerImageName createImage(String originalImage, String tag) {
             .withTag(tag);
 
         // push the image to the registry
-        client.tagImageCmd(dummyImageId, imageName.asCanonicalNameString(), tag).exec();
+        client.tagImageCmd(dummyImageId, imageName.getUnversionedPart(), tag).exec();
 
         client
             .pushImageCmd(imageName.asCanonicalNameString())