Build on Ubuntu without Docker for the sake of documentation

.github/workflows/ubuntu.yml

@@ -0,0 +1,30 @@
+name: Build AppImageUpdate on Ubuntu without Docker
+
+on:
+  pull_request
+
+jobs:
+  build:
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Install dependencies
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends software-properties-common
+          sudo add-apt-repository ppa:ubuntu-toolchain-r/test
+          sudo add-apt-repository -y ppa:beineri/opt-qt-5.15.2-focal
+          sudo apt-get update
+          sudo apt-get install -y g++-10 qt515base qt515wayland libgl1 libdrm-dev mesa-common-dev build-essential libssl-dev autoconf automake libtool wget vim-common desktop-file-utils pkgconf libgpgme-dev libglib2.0-dev libcairo2-dev librsvg2-dev libfuse-dev git libcurl4-openssl-dev argagg-dev libgcrypt20-dev libboost-dev libarchive-dev libzstd-dev nlohmann-json3-dev cmake
+
+      - name: Build
+        run: |
+          export CXX=g++-10
+          mkdir build
+          cd build
+          cmake .. -DCMAKE_INSTALL_PREFIX=/usr -DBUILD_TESTING=OFF
+          make -j$(nproc)
+          sudo make install
+          cd ..