CI: Add initial workflow to deploy image and test it.

.github/workflows/deploy-test.yml

@@ -0,0 +1,53 @@
+name: Deploy main & test builds
+
+on:
+  push:
+    branches: [main, ci]
+
+jobs:
+  deploy:
+    name: Deploy main tag image
+    runs-on: ubuntu-latest
+    if: "!contains(github.event.head_commit.message, 'ci-skip') && !contains(github.event.head_commit.message, 'skip-ci')"
+    steps:
+      - uses: actions/checkout@v2
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Build image
+        run: docker build -t ghcr.io/mu-editor/mu-appimage:main .
+      - name: Push master tag to ghcr.io
+        run: docker push ghcr.io/mu-editor/mu-appimage:main
+
+  build-mu-appimage:
+    name: Build Mu AppImage with main tag image
+    needs: deploy
+    runs-on: ubuntu-latest
+    container: ghcr.io/mu-editor/mu-appimage:main
+    steps:
+      - name: Check Versions
+        run: |
+          uname -a
+          python -c "import sys; print(sys.version)"
+          python -c "import platform, struct; print(platform.machine(), struct.calcsize('P') * 8)"
+          python -c "import sys; print(sys.executable)"
+          python -m pip --version
+          pip --version
+          pip list --verbose
+      - name: Clone Mu
+        run: |
+          git clone https://github.com/mu-editor/mu.git
+          cd mu
+          git checkout -b origin/docker-appimage
+      - name: Install Mu test dependencies
+        run: |
+          cd mu
+          pip install .[tests]
+          pip list
+      - name: Build Mu AppImage
+        run: |
+          cd mu
+          xvfb-run make linux