ci: Add test-ubuntu-20.04 job (#270)

- Bumps mdBook from 0.3.6 to 0.3.7
- Add test job for ubuntu 20.04

cf. #268

.github/workflows/test.yml

@@ -89,7 +89,7 @@ jobs:
         if: github.ref == 'refs/heads/master'
         uses: peaceiris/actions-mdbook@v1
         with:
-          mdbook-version: '0.3.6'
+          mdbook-version: '0.3.7'
 
       - name: Build site
         if: github.ref == 'refs/heads/master'
@@ -170,3 +170,82 @@ jobs:
           user_name: 'github-actions[bot]'
           user_email: 'github-actions[bot]@users.noreply.github.com'
           # commit_message: ${{ github.event.head_commit.message }}
+
+  test-ubuntu-2004:
+    runs-on: 'ubuntu-18.04'
+    if: contains(github.event.head_commit.message, '[skip ci]') == false
+    container:
+      image: 'ubuntu:20.04'
+    steps:
+      - name: Install deps
+        run: |
+          apt update
+          apt install -y software-properties-common
+          add-apt-repository ppa:git-core/ppa
+          apt update
+          apt install -y \
+            git \
+            ssh
+
+      - uses: actions/checkout@v2
+
+      - name: Dump GitHub context
+        env:
+          GITHUB_CONTEXT: ${{ toJson(github) }}
+        run: echo "${GITHUB_CONTEXT}"
+
+      - name: Read .nvmrc
+        run: echo "::set-output name=NVMRC::$(cat .nvmrc)"
+        id: nvm
+
+      - name: Setup Node
+        uses: actions/setup-node@v1
+        with:
+          node-version: '${{ steps.nvm.outputs.NVMRC }}'
+
+      - run: npm i -g npm
+
+      - name: Dump version
+        run: |
+          node -v
+          npm --version
+          git --version
+
+      - run: npm ci
+      - run: npm test
+
+      - name: Run ncc
+        run: npm run build
+
+      - name: Remove lint-staged husky
+        if: github.ref == 'refs/heads/master'
+        run: |
+          npm uninstall lint-staged husky
+          git checkout package-lock.json package.json
+
+      - name: Setup mdBook
+        if: github.ref == 'refs/heads/master'
+        uses: peaceiris/actions-mdbook@v1
+        with:
+          mdbook-version: '0.3.7'
+
+      - name: Build site
+        if: github.ref == 'refs/heads/master'
+        working-directory: ./test_projects/mdbook
+        run: mdbook build
+
+      - name: Deploy
+        if: github.ref == 'refs/heads/master'
+        uses: ./
+        with:
+          # deploy_key: ${{ secrets.ACTIONS_DEPLOY_KEY }}
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          publish_branch: gh-pages-ubuntu-20.04
+          publish_dir: ./test_projects/mdbook/book
+          # external_repository: ''
+          allow_empty_commit: true
+          # keep_files: true
+          # force_orphan: true
+          user_name: 'github-actions[bot]'
+          user_email: 'github-actions[bot]@users.noreply.github.com'
+          # commit_message: ${{ github.event.head_commit.message }}