fix: fix helm-docs and do various helm improvements

- Add valuesFiles option to HelmTemplate config for passing additional values files to helm template command
- Remove redundant -f values.yaml flag from helm template (chart's default values.yaml is used automatically)
- Remove --template-files flag with typo from helm-docs (default README.md.gotmpl is correct)
- Add buildx setup step to helm workflow to fix CI hang (was missing remote buildkit driver)
- Extract SetupBuildxStep() to avoid code duplication
- Add test helm chart to validate helm CI flow
- Fix the workdir of helm-docs

Signed-off-by: Utku Ozdemir <utku.ozdemir@siderolabs.com>

.github/workflows/helm.yaml

@@ -0,0 +1,119 @@
+# THIS FILE WAS AUTOMATICALLY GENERATED, PLEASE DO NOT EDIT.
+#
+# Generated on 2026-01-30T10:33:51Z by kres ae0b9fab-dirty.
+
+concurrency:
+  group: helm-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+"on":
+  push:
+    tags:
+      - v*
+  pull_request:
+    branches:
+      - main
+      - release-*
+    paths:
+      - test/**
+name: helm
+jobs:
+  default:
+    permissions:
+      actions: read
+      contents: write
+      id-token: write
+      issues: read
+      packages: write
+      pull-requests: read
+    runs-on:
+      group: generic
+    steps:
+      - name: gather-system-info
+        id: system-info
+        uses: kenchan0130/actions-system-info@59699597e84e80085a750998045983daa49274c4 # version: v1.4.0
+        continue-on-error: true
+      - name: print-system-info
+        run: |
+          MEMORY_GB=$((${{ steps.system-info.outputs.totalmem }}/1024/1024/1024))
+
+          OUTPUTS=(
+            "CPU Core: ${{ steps.system-info.outputs.cpu-core }}"
+            "CPU Model: ${{ steps.system-info.outputs.cpu-model }}"
+            "Hostname: ${{ steps.system-info.outputs.hostname }}"
+            "NodeName: ${NODE_NAME}"
+            "Kernel release: ${{ steps.system-info.outputs.kernel-release }}"
+            "Kernel version: ${{ steps.system-info.outputs.kernel-version }}"
+            "Name: ${{ steps.system-info.outputs.name }}"
+            "Platform: ${{ steps.system-info.outputs.platform }}"
+            "Release: ${{ steps.system-info.outputs.release }}"
+            "Total memory: ${MEMORY_GB} GB"
+          )
+
+          for OUTPUT in "${OUTPUTS[@]}";do
+            echo "${OUTPUT}"
+          done
+        continue-on-error: true
+      - name: checkout
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # version: v6.0.1
+      - name: Unshallow
+        run: |
+          git fetch --prune --unshallow
+      - name: Set up Docker Buildx
+        id: setup-buildx
+        uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # version: v3.12.0
+        with:
+          driver: remote
+          endpoint: tcp://buildkit-amd64.ci.svc.cluster.local:1234
+        timeout-minutes: 10
+      - name: Install Helm
+        uses: azure/setup-helm@1a275c3b69536ee54be43f2070a358922e12c8d4 # version: v4.3.1
+      - name: Install cosign
+        if: github.event_name != 'pull_request'
+        uses: sigstore/cosign-installer@faadad0cce49287aee09b3a48701e75088a2c6ad # version: v4.0.0
+      - name: Login to registry
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@5e57cd118135c172c3672efd75eb46360885c0ef # version: v3.6.0
+        with:
+          password: ${{ secrets.GITHUB_TOKEN }}
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+      - name: Lint chart
+        if: github.event_name == 'pull_request'
+        run: |
+          helm lint test/test-helm-chart
+      - name: Template chart
+        if: github.event_name == 'pull_request'
+        run: |
+          helm template -f test/test-helm-chart/ci-values.yaml test-helm-chart test/test-helm-chart
+      - name: Install unit test plugin
+        if: github.event_name == 'pull_request'
+        run: |
+          make helm-plugin-install
+      - name: Unit test chart
+        if: github.event_name == 'pull_request'
+        run: |
+          make chart-unittest
+      - name: Generate schema
+        if: github.event_name == 'pull_request'
+        run: |
+          make chart-gen-schema
+      - name: Generate docs
+        if: github.event_name == 'pull_request'
+        run: |
+          make helm-docs
+      - name: Check dirty
+        if: github.event_name == 'pull_request'
+        run: |
+          make check-dirty
+      - name: helm login
+        if: startsWith(github.ref, 'refs/tags/')
+        env:
+          HELM_CONFIG_HOME: /var/tmp/.config/helm
+        run: |
+          helm registry login -u ${{ github.repository_owner }} -p ${{ secrets.GITHUB_TOKEN }} ghcr.io
+      - name: Release chart
+        if: startsWith(github.ref, 'refs/tags/')
+        env:
+          HELM_CONFIG_HOME: /var/tmp/.config/helm
+        run: |
+          make helm-release