GitLab CI - :image needed if runner runs on a vm with pre-installed docker-compose

Question

I'm wondering that most of tutorials for the configuration of .gitlab-ci.yml use the image: docker or image: docker/compose.

In my case we have a pre-installed docker and docker-compose on our virtual machine (Linux).

1. So is it necessary to use a image definition?
2. In other cases they often use the dind (Docker-in-Docker) functionality, is that necessary in my case?
3. If not, when do i use it/is it useful?

Answer 1

1. So is it necessary to use a image definition?

No, as mentioned in "Using Docker images"

GitLab CI/CD in conjunction with GitLab Runner can use Docker Engine to test and build any application.

When used with GitLab CI/CD, Docker runs each job in a separate and isolated container using the predefined image that’s set up in .gitlab-ci.yml.

So you can use any image you need for your job to run.

2. In other cases they often use the dind (Docker-in-Docker) functionality, is that necessary in my case?
   3. If not, when do i use it/is it useful?

As documented in "Building Docker images with GitLab CI/CD ", this is needed if your job is to build a docker image (as opposed to use an existing docker image)