feat: add lint_config input for custom yamllint rules

[Bisman-Singh]

Adds a lint_config input that lets users provide a custom yamllint config file. When set, it replaces the default lintconf.yaml in CT_CONFIG_DIR so ct lint picks it up automatically.

Usage:

- uses: helm/chart-testing-action@v2
  with:
    lint_config: .github/lintconf.yaml

This lets users customize yamllint rules (e.g. min-spaces-from-content, line-length) without needing to pass --lint-conf to every ct lint call.

When lint_config is not set, behavior is unchanged.

Fixes #176

[Bisman-Singh]

@cpanato could you review this when you have a moment?

[Bisman-Singh]

Friendly ping on this one — it's been open a while.

Recap: this adds an optional lint_config input so a chart repo can pass its own yamllint rules to chart-testing, instead of being limited to the action's defaults. It's additive (no behaviour change when the input is unset), DCO is signed, and the branch is green.

Happy to rebase onto main or adjust the approach if you'd prefer it done differently. No rush — just flagging it in case it slipped off the queue. 🙂

[cpanato]

@Bisman-Singh can you rebase to fix the conflicts? thanks

[Bisman-Singh]

Rebased on main, thanks. The conflict was the new github_token input sitting where lint_config was added. While testing I found the old step wrote the config into the shared tool cache, which on self-hosted runners leaks into later jobs that never set lint_config, so it now passes the file through CT_LINT_CONF instead. That change is in its own commit so it's easy to review separately.

[cpanato]

can we add a ci job test to exercise this feature? thanks again

[Bisman-Singh]

Added test_ct_action_with_lint_config. It lints the test chart with a config that only enables document-start, so it fails only when ct actually picks up the file, and it also checks that a missing file fails the action.