feat(docs): add cache sections to gitlab docker kubernetes executors

[Fral738]

No description provided.

[alexey-igrychev]

This volume is also related to caching and requires running werf host cleanup.
Let’s stick to a single approach that uses caching — users can decide which parts to disable if needed.
It would also be helpful to add a comment to each volume for clarity.

[ilya-lesikov]

I'm kinda confused. Initially this file was an instruction for any kind of GitLab Runner. Now it's only for GitLab Runner in Docker. Is GitLab Runner in Docker really a requirement? Note that Executor is a different thing, you can have GitLab Runner on a bare OS, but with Docker Executor.

[Fral738]

Originally, the documentation seemed to support both scenarios:

1. GitLab Runner installed directly on the host OS
2. GitLab Runner running inside a Docker container

However, the cache configuration provided didn't actually work in either case.

I'm now updating the documentation to include a working cache configuration, but the setup differs depending on how GitLab Runner itself is deployed (not just the executor type).

Should I provide instructions for both cases, or do we want to support only one of them going forward?

[ilya-lesikov]

How much it differs? Can we just say "go to GitLab docs to install GitLab Runner the way you want", then explain extra steps needed for different types of the GitLab Runners? Maybe we can even explain it in a generic way, like there was instructions on what to add to config.toml without specifying how to update the config.toml (depends on how the Runner installed).

[Fral738]

Summary:
We'll use named Docker volumes to mount cache directories in the GitLab Runner configuration. This approach works consistently regardless of how the Runner itself is deployed — whether it's running directly on the host or inside a Docker container. It also avoids relying on $HOME or UID-specific paths, making the setup more robust and portable.

As a result, the documentation can stay generic: we’ll point users to the [official GitLab docs](https://docs.gitlab.com/runner/install/) for installation and registration steps, and only highlight the specific additions needed in config.toml to support caching and other features.

[ilya-lesikov]

Needs changes

[ilya-lesikov]

How much it differs? Can we just say "go to GitLab docs to install GitLab Runner the way you want", then explain extra steps needed for different types of the GitLab Runners? Maybe we can even explain it in a generic way, like there was instructions on what to add to config.toml without specifying how to update the config.toml (depends on how the Runner installed).