You can define the image to use per job.

For instance:

before_script:
   - apt-get install --yes cmake libmatio-dev libblas-dev libsqlite3-dev libcurl4-openssl-dev
   - apt-get install --yes libarchive-dev liblzma-dev

build:precise:
  image: precise:base
  script:
    - mkdir build/
    - cd build
    - cmake -D CMAKE_BUILD_TYPE=Debug ../
    - make

build:trusty:
  image: trusty:base
  script:
    - mkdir build/
    - cd build
    - cmake -D CMAKE_BUILD_TYPE=Debug ../
    - make

Since multi architecture/platform tags of a Docker image have different digests, You can pull a Docker image using its digest (instead of using tags) to pull the desired architecture/platform.

Here is an example of multi architecture/platform tag of a Docker image (Ubuntu) in Docker Hub: As you can see, 20.04 is a multi architecture tag and there are different digests for each of architectures in the tag.
If you run command docker pull ubuntu:20.04 it will pull all architectures.
But command
docker pull ubuntu@sha256:55e5613c8c7bcd8044aaf09d64d20518964a0d7a6e41af129f95b731301c2659 will pull just linux/arm/v7.

As I tried, it is possible to use digest in .gitlab-ci.yml:

job_1:
  image: ubuntu@sha256:55e5613c8c7bcd8044aaf09d64d20518964a0d7a6e41af129f95b731301c2659
  script:
    - ...

job_2:
  image: alpine@sha256:71465c7d45a086a2181ce33bb47f7eaef5c233eace65704da0c5e5454a79cee5
  script:
    - ...

The only thing needed to access docker images from another project on the same gitlab instance is that your project is allowed for Token Access on the project with the image.

No other configuration like docker login is needed. Just use image: $CI_REGISTRY/other/project:tag

job-in-project1:
  stage: test
  image: $CI_REGISTRY/group-b/otherproject:latest
  script:
    - command to run with docker image

Configuration:

• Go to the project wich contains the image in its Container Registry. e.g. group-b/otherproject
• Open Settings > CI/CD
• Expand "Token Access"
• in Allow CI job tokens from the following projects to access this project click "Add Project"

Docker uses the host's kernel. As you can see by your uname -a output, it gives the runner's machine info. That's why regardless of the container you get the same output. You can even try it on your machine.

https://stackoverflow.com/a/31012367/4551937

Regarding Gitlab Runner, your service is independent. It will be attached and provided to your job calling it, as if it was running beside it.

Your job will use the image you specify with the image tag to run the commands listed in script (you can chain them). If no image is specified, it will take the default one defined in the runner configuration (probably Ubuntu).