test: qualify Docker image with version so that client version and server version match (#23838) [2.40] (#23848)

Author: netroms

dhis-2/dhis-test-web-api/src/test/java/org/hisp/dhis/webapi/controller/RouteControllerTest.java

@@ -123,7 +123,7 @@ public DhisConfigurationProvider dhisConfigurationProvider() {
   @BeforeAll
   public static void beforeAll() {
     upstreamMockServerContainer =
-        new GenericContainer<>("mockserver/mockserver")
+        new GenericContainer<>("mockserver/mockserver:5.15.0")
             .waitingFor(new HttpWaitStrategy().forStatusCode(404))
             .withExposedPorts(1080);
     upstreamMockServerContainer.start();