In GitLab pipeline I see that each stage runs in its own container. How can I run all stages/jobs in the same environment? I don't need any specific docker image at all, just runner environment is fine.
If you self-host your runner, you can use the shell executor which executes your job scripts directly on the runner host without any container.
When using the shell executor, any image: key defined in any of your CI YAML files will just be ignored.