Switch gamecube-iso to run inside extremscorner/libogc2 container

Avoids devkitPro apt server rate-limiting in CI by using the pre-built
ghcr.io/extremscorner/libogc2 container (same base as the Dockerfile).
Toolchain is already installed; only apt/dkp extras are added per-run.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/build.yml

@@ -137,11 +137,33 @@ jobs:
 
   build-gamecube-iso:
     runs-on: ubuntu-latest
+    container:
+      image: ghcr.io/extremscorner/libogc2:latest
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
-      - name: Setup devkitPro
-        uses: ./.github/actions/setup-devkitpro
+      - name: Install additional dependencies
+        run: |
+          apt-get update
+          apt-get install -y \
+            python3 \
+            python3-pip \
+            python3-polib \
+            python3-pil \
+            python3-dotenv \
+            python3-pyqt5 \
+            python3-opengl \
+            xorriso
+          dkp-pacman -Syu --noconfirm
+          dkp-pacman -S --needed --noconfirm \
+            gamecube-sdl2 \
+            ppc-liblzma \
+            ppc-libzip \
+            gamecube-tools \
+            ppc-libmad \
+            ppc-zlib-ng \
+            ppc-bzip2 \
+            ppc-zstd
       - name: Build GameCube ISO
         run: ./scripts/build-gamecube-iso.sh
       - name: Copy output files