Use git clone for Flutter SDK to support arm64

Switch from downloading pre-built Flutter SDK tarballs to cloning from
git. Flutter only provides x64 Linux tarballs, but cloning from git
allows Flutter to bootstrap itself with the correct Dart SDK for any
host architecture (amd64 or arm64).

Also reduces image size from ~4GB to ~1.7GB for Android variant.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Dockerfile

@@ -8,21 +8,12 @@ LABEL org.opencontainers.image.version="${FLUTTER_VERSION}"
 
 USER 0
 
-# Download Flutter SDK and strip unnecessary files in a single layer
-RUN curl -fsSL "https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_${FLUTTER_VERSION}-stable.tar.xz" \
-      -o /tmp/flutter.tar.xz && \
-    tar xf /tmp/flutter.tar.xz -C /opt && \
-    rm /tmp/flutter.tar.xz && \
-    rm -rf /opt/flutter/dev \
-           /opt/flutter/examples \
-           /opt/flutter/bin/cache/artifacts/engine/android-* \
-           /opt/flutter/bin/cache/artifacts/engine/linux-* && \
+# Clone Flutter SDK from git (supports both amd64 and arm64)
+RUN git clone --depth 1 --branch ${FLUTTER_VERSION} https://github.com/flutter/flutter.git /opt/flutter && \
+    git config --global --add safe.directory /opt/flutter && \
+    rm -rf /opt/flutter/dev /opt/flutter/examples && \
     chown -R 65532:65532 /opt/flutter
 
-# Mark git directory as safe and compact git history
-RUN git config --global --add safe.directory /opt/flutter && \
-    cd /opt/flutter && git gc --prune=all
-
 USER 65532
 
 # Configure for web-only (disable everything else)