Include build action for an AppImage

Credit for the icon to the Alire project:
https://github.com/alire-project/alire/blob/master/resources/alr.png

.github/workflows/main.yml

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+# Build using Alire.
+name: Build
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  build:
+    name: CI on ${{ matrix.os }}
+
+    runs-on: ${{ matrix.os }}
+
+    strategy:
+      matrix:
+        os: [windows-latest, ubuntu-latest]
+
+    steps:
+       - name: Checkout
+         uses: actions/checkout@v2
+       - name: ada-actions/toolchain
+         uses: ada-actions/toolchain@ce2021
+         with:
+           distrib: community
+           target: native
+       - name: alire-project/setup-alire
+         uses: alire-project/setup-alire@v1
+         with:
+           version: 1.2.1
+       - name: Update apt
+         if: runner.os == 'Linux'
+         run: sudo apt-get update
+       - name: Update
+         run: alr index --update-all
+       - name: Build
+         run: alr --non-interactive build
+
+  release:
+    name: Continuous AppImage
+    runs-on: ubuntu-20.04
+    steps:
+       - name: Checkout
+         uses: actions/checkout@v2
+       - name: ada-actions/toolchain
+         uses: ada-actions/toolchain@ce2021
+         with:
+           distrib: community
+           target: native
+       - name: alire-project/setup-alire
+         uses: alire-project/setup-alire@v1
+       - name: Update apt
+         run: sudo apt-get update
+       - name: Update
+         run: alr index --update-all
+       - name: Build
+         run: alr --non-interactive build
+       - name: Build AppImage
+         run: VERSION=${GITHUB_BASE_REF:-${GITHUB_REF#refs/heads/}} make AppImage
+      - name: Release
+        uses: softprops/action-gh-release@v1
+        with:
+          files: Open_URL*.AppImage
+          prerelease: true
+          tag_name: continuous

Makefile

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+MAIN = open_url
+DESTDIR = /opt/$(MAIN)
+
+.PHONY: all install AppImage
+
+all:
+	alr build
+
+install: all
+	alr exec -P -- gprinstall --prefix=$(DESTDIR) --create-missing-dirs --mode=usage -f
+	cp -p $(MAIN).png $(DESTDIR)/
+
+AppImage:
+	rm -rf AppDir
+	make install DESTDIR=AppDir/usr
+	wget -nv -c https://github.com/linuxdeploy/linuxdeploy/releases/download/continuous/linuxdeploy-x86_64.AppImage
+	chmod +x linuxdeploy-x86_64.AppImage
+	./linuxdeploy-x86_64.AppImage \
+	--executable bin/$(MAIN) \
+	--desktop-file $(MAIN).desktop --icon-file=AppDir/usr/$(MAIN).png \
+	--appdir AppDir --output appimage

open_url.desktop

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+[Desktop Entry]
+Name=Open URL
+Comment=Open *.url files under Linux
+Exec=open_url
+Icon=open_url
+Terminal=false
+Type=Application
+Categories=Utility
+