CI: Use GitLab executors Docker socket directly

CI improvements

• Docker 20.10.12
• Use the host Docker socket directly => better performance and stability, Docker daemon already configured properly
• You can transparently pull image from private ECRs inside the AWS account (we want to mirror Docker image from gitlab.com to avoid bandwidth billing)

Context

The devops team refreshed the GitLab runners/executors to run Docker 20.10.12 and match the version used everywhere

Testing

• Sample test job using the Docker socket directly and showing Docker environment information: https://gitlab.com/tezos/tezos/-/jobs/2205173480
• The branch has docker in the name, Docker images are built