Set up GitHub actions for CI and release builds

.github/workflows/pr-build.yml

@@ -0,0 +1,20 @@
+name: Pull Request Build
+
+on:
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: 2.7
+        bundler-cache: true # runs 'bundle install' and caches installed gems automatically
+
+    - name: Build book
+      run: bundle exec rake book:build

.github/workflows/release-on-merge.yml

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+name: Release on push to master
+
+on:
+  push:
+    branches: [ master ]
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+
+    - name: Compute tag name
+      id: compute-tag
+      run: |
+        echo Computing next tag number
+        LASTPATCH=$(git describe --tags | cut -d- -f1 | cut -d. -f3)
+        PATCH=$(($LASTPATCH+1))
+        echo "::set-output name=tagname::2.1.${PATCH}"
+
+    - name: Set up Ruby
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: 2.7
+        bundler-cache: true # runs 'bundle install' and caches installed gems automatically
+
+    - name: Build release assets
+      run: bundle exec rake book:build
+
+    - name: Create release
+      uses: ncipollo/release-action@v1
+      with:
+        token: ${{ secrets.GITHUB_TOKEN }}
+        tag: ${{ steps.compute-tag.outputs.tagname }}
+        commit: master
+        artifacts: './progit.epub,./progit.mobi,./progit.pdf,./progit.html'