[Docs] Add Kubernetes APT mirrors configuration option

[Michaelvll]

Description

This PR adds a new kubernetes.apt_mirrors configuration option that allows users to override the APT mirrors used when installing packages on Kubernetes pods.

Changes

• Added apt_mirrors configuration parameter to the Kubernetes section of the config schema
• Users can now specify a list of APT mirror hostnames (without scheme/path) that will be tried in order
• Setting to an empty list [] disables fallback mirrors entirely
• When unset, SkyPilot uses built-in fallback mirrors (mirrors.wikimedia.org, mirror.umd.edu)
• The option can be set globally or per-context via context_configs
• Added comprehensive documentation with examples and explanation of the feature

Example Usage

kubernetes:
  apt_mirrors:
    - mirror.math.princeton.edu
    - mirrors.kernel.org

Testing

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)

https://claude.ai/code/session_01BMUMi6V6LebBXsid2RuKj1

[Michaelvll]

Add docs for #9553

[gemini-code-assist]

Code Review

This pull request adds documentation for the kubernetes.apt_mirrors configuration setting, which allows users to specify a list of hostnames to be used as fallback APT mirrors for Kubernetes pods. The review feedback suggests clarifying whether this field overrides the entire system mirror list or only the fallback mirrors injected by SkyPilot. Additionally, it is recommended to explicitly mention that URL schemes like http:// should be omitted and to clarify if custom ports are supported.

[gemini-code-assist]

The documentation states that setting apt_mirrors to [] disables fallback mirrors entirely. It would be beneficial to clarify whether this field overrides the entire list of APT mirrors (including the primary ones in /etc/apt/sources.list) or only the fallback mirrors that SkyPilot injects. Based on the description, it seems to only affect the fallback list, so specifying this explicitly would avoid confusion for users with custom images that already have pre-configured mirrors.

[gemini-code-assist]

The instruction 'Hostnames only (no scheme/path)' is clear, but it might be worth explicitly mentioning that this implies http:// or https:// should be omitted, as users frequently include the scheme when copying mirror URLs. Additionally, clarifying if ports are supported (e.g., my-mirror:8080) would be helpful for users with internal repository managers.