How to reach a running container within gitlab runner

I've been experiencing an issue and even with all I could read on the web, I keep on getting stuck...

I've developed an "abstraction" for MongoDB and I've made some Unit tests and some E2E tests. For the "End to End" tests, I need MongoDB to run within my pipeline. Yesterday I was on CloudBuild but today I am on Gitlab and... I am not able anymore to run these tests...

---

I have the following:

gitlab-ci.yml

image: docker:latest

services:
  - docker:dind

variables:
  DOCKER_TLS_CERTDIR: ""
  DOCKER_HOST: "tcp://docker:2375"

stages:
  - test


.anchors:
  - &start-mongodb-docker-compose
      apk add --no-cache docker-compose
      && docker-compose -f e2e_tests/docker-compose.yml up --build -d


integration-tests:
  image: golang:1.16
  stage: test
  before_script:
    - *start-mongodb-docker-compose
  script:
    - make test
  only:
    - master

e2e_tests/docker-compose.yml

version: "3.7"

services:
  mongodb:
    container_name: MongoDB_E2E
    image: mongo
    restart: always
    ports:
      - "27017:27017"

Makefile

PROJECT_NAME="yyyyyy"
PKG="gitlab.com/xxxxxx/$(PROJECT_NAME)"

.PHONY: all

all: test
test: unit-tests e2e-tests

unit-tests: ## Run unittests
	@go test -v ./... --tags=unit

e2e-tests: ## Run end-to-end tests
	@go test -v ./... -p 1 --tags=e2e

coverage: ## Generate global code coverage report
	@go test -v -coverprofile .testCoverage.txt

---

As you can see, I already tried some from the web, such as DOCKER_HOST but.. nothing worked as of now. If anyone has an idea, I would love to hear about it. I also tried to set the network as I did for CloudBuild but it didn't work either...

Thanks for any help!

Max