Selenium tests works in local but not in Gitlab CI - selenium

Question

I am currently working on a Java Spring Boot project which involves a classic backend/frontend architecture. I am trying to write some basic integration test by using the Selenium WebDriver.
The problem is that the tests I write pass without any problem on my local development machine but do not pass when I run them thorugh the continuous integration setup (Gitlab CI).
The code of the example test is the following:
#RunWith(SpringRunner.class)
#SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
#ActiveProfiles("test")
public class ExampleTest {
#LocalServerPort
private int port;
WebDriver wd;
#Test
public void successfulLogin(){
String url = "http://localhost:" + port;
wd = new HtmlUnitDriver();
wd.manage().timeouts().pageLoadTimeout(30, TimeUnit.SECONDS);
wd.get(url);
}
}
The relative gitlab-ci.yml portion is:
stages:
- test
maven-test:
image: maven:3.5.0-jdk-8
stage: test
script: "mvn test -B -Dmaven.repo.local=/root/.m2/"
only:
- master
The CI has a single runner (version 9.5.0) with concurrency 1. It uses the docker executor with the docker:stable image. I don't know if it is needed, but it is running in priviledged mode.
When running the tests on the CI environment, they fail with the following error:
org.openqa.selenium.TimeoutException: java.net.SocketTimeoutException: Read timed out
I have tried both with url = "http://localhost:" + port and url = InetAddress.getLocalHost().getHostName() + ":" + port, both passed locally, none passed in the CI environment.
What am I doing wrong?
EDIT: Alfageme suggested a more accurate testing methodology. On the same server on which the CI is running, I cloned my repository with git clone and then run the following command:
sudo gitlab-runner exec docker maven-test
The test passed without any problem. I am really running out of ideas, does someone have any?

Answer 1

I am not exactly sure why, but clearing the various runner-XXXXX-project-21-concurrent-0-cache-XXXXXXXXXXXXXXXXXXX docker containers on the CI machine seemed to have solved the issue.
EDIT: This fixed the issue only momentarely. The problem happened again, this time clearing the cache voulmes did not help. Anyone has any suggestion?