Add build script for macos dmgs.

.github/workflows/build-dmg.yml

@@ -0,0 +1,44 @@
+name: Build DMG
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  build-dmg:
+    name: Build (macOS, DMG)
+    runs-on: macos-14
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Qt
+        uses: jurplel/install-qt-action@v4
+        with:
+          version: 6.10.*
+          modules: qtshadertools
+          setup-python: false
+          cache: true
+
+      - name: Build DMG
+        run: |
+          JOBS="$(sysctl -n hw.ncpu)"
+          ./scripts/build-dmg.sh
+
+      - name: Collect artifact
+        run: |
+          mkdir -p release
+          mv ./*.dmg release/
+
+      - name: Attestation
+        uses: actions/attest-build-provenance@v1
+        with:
+          subject-path: ./release/*
+
+      - name: Upload artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: cool-retro-term-dmg
+          path: ./release/*