[CI] Fix test fixture caching in ci image baking (#140358) (#140368)

This should fix the vm image jobs for baking our ci images that broken due to unability to cache docker
test fixtures

Author: breskeby

test/fixtures/testcontainer-utils/src/main/java/org/elasticsearch/test/fixtures/testcontainers/DockerEnvironmentAwareTestContainer.java

@@ -23,7 +23,7 @@
 
 import static org.elasticsearch.test.fixtures.testcontainers.DockerAvailability.assumeDockerIsAvailable;
 
-public class DockerEnvironmentAwareTestContainer extends GenericContainer<DockerEnvironmentAwareTestContainer>
+public abstract class DockerEnvironmentAwareTestContainer extends GenericContainer<DockerEnvironmentAwareTestContainer>
     implements
         TestRule,
         CacheableTestFixture {