Meta: migrate to GitHub Actions

This takes advantage of the new CI build strategy in
whatwg/html-build#254.

Part of whatwg/meta#173.

.github/workflows/build.yml

@@ -0,0 +1,47 @@
+name: Build
+on:
+  pull_request:
+    branches:
+    - master
+  push:
+    branches:
+    - master
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-20.04
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 2
+    - name: build
+      run: |
+        docker run --rm --interactive --tty \
+                --mount "type=bind,source=.,destination=/whatwg/html,readonly=1" \
+                --env "HTML_SOURCE=/whatwg/html" \
+                --mount "type=bind,source=$GITHUB_WORKSPACE/output,destination=/whatwg/output" \
+                --env "HTML_OUTPUT=/whatwg/output" \
+                whatwg/html-build
+    - name: deploy
+      if: ${{ github.event_name == 'push' && github.ref == 'refs/heads/master' }}
+      env:
+        SERVER: ${{ secrets.MARQUEE_SERVER }}
+        SERVER_PUBLIC_KEY: ${{ secrets.MARQUEE_PUBLIC_KEY }}
+        SERVER_DEPLOY_KEY: ${{ secrets.MARQUEE_DEPLOY_KEY }}
+      run: |
+        eval "$(ssh-agent -s)"
+        echo "$SERVER_DEPLOY_KEY" | ssh-add -
+        mkdir -p ~/.ssh/ && echo "$SERVER $SERVER_PUBLIC_KEY" > ~/.ssh/known_hosts
+
+        # Sync, including deletes, but ignoring the stuff we'll deploy below, so that we don't delete them.
+        echo "Deploying build output..."
+        # --chmod=D755,F644 means read-write for user, read-only for others.
+        rsync --archive --chmod=D755,F644 --compress --verbose \
+              --delete --exclude="commit-snapshots" --exclude="review-drafts" \
+              "$GITHUB_WORKSPACE/output/" "deploy@$SERVER:/var/www/html.spec.whatwg.org"
+
+        # Now sync a commit snapshot and a review draft, if any
+        echo ""
+        echo "Deploying Commit Snapshot and Review Drafts, if any..."
+        rsync --archive --chmod=D755,F644 --compress --verbose \
+              "$GITHUB_WORKSPACE/output/commit-snapshots" "$GITHUB_WORKSPACE/output/review-drafts" "deploy@$SERVER:/var/www/html.spec.whatwg.org"