Allow setting the MTU of nested Docker daemon in DinD mode

[tsuna]

When running DinD it's common to need a lower MTU on the nested Docker bridge.

Use a default of 1450 as it's generally safer than not doing anything, and pretty harmless if done when not needed.

[tsuna]

Ping. Please :)

[Copilot]

Pull Request Overview

This PR adds MTU configuration support for the nested Docker daemon when running in DinD (Docker-in-Docker) mode. The change allows users to customize the MTU setting for better network compatibility in nested container environments, with a safe default of 1450.

Key Changes:

• Added MTU configuration option for DinD mode with a default value of 1450
• Updated Helm chart template to pass MTU setting to dockerd command

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
charts/gha-runner-scale-set/values.yaml	Added documentation for the new containerMode.dind.mtu configuration option
charts/gha-runner-scale-set/templates/_helpers.tpl	Added --mtu flag to dockerd arguments with configurable value

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

[nitpick] The comment states 'Defaults to 1450 in DinD mode' but doesn't clarify what happens in non-DinD modes. Consider clarifying that this setting only applies when type: \"dind\" is set, or that it has no effect in other modes.

Suggested change

	# mtu: 1450 ## Optional. Defaults to 1450 in DinD mode
	# mtu: 1450 ## Optional. Only applies when containerMode.type is "dind". Defaults to 1450 in DinD mode. Has no effect in other modes.

[benoit-nexthop]

@Draco7587 you approved this change 3 weeks ago but some workflows remained waiting for approval. Would it be possible to let those run so this PR can merge? Thanks in advance.