Add Android and Linux desktop SDK variants with matrix pipelines

Adds Dockerfile.android (Flutter + Android SDK/JDK 17) and
Dockerfile.linux (Flutter + clang/cmake/GTK3 for desktop builds).
Publish and Scout pipelines now use matrix strategy to build all
three variants in parallel. Registry secrets updated to
REGISTRY_USERNAME/REGISTRY_PASSWORD. Update-check adds explicit
stable channel filter.

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

Dockerfile.linux

@@ -0,0 +1,55 @@
+FROM debian:bookworm-slim
+
+ARG FLUTTER_VERSION=3.38.9
+
+LABEL org.opencontainers.image.title="gpb-flutter-sdk-linux"
+LABEL org.opencontainers.image.description="Flutter SDK for Linux desktop CI builds"
+LABEL org.opencontainers.image.version="${FLUTTER_VERSION}"
+
+# Install dependencies for Flutter + Linux desktop builds
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    curl \
+    git \
+    unzip \
+    xz-utils \
+    ca-certificates \
+    clang \
+    cmake \
+    ninja-build \
+    pkg-config \
+    libgtk-3-dev \
+    liblzma-dev \
+    libstdc++-12-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+ENV FLUTTER_HOME=/opt/flutter
+ENV PATH="${FLUTTER_HOME}/bin:${FLUTTER_HOME}/bin/cache/dart-sdk/bin:${PATH}"
+
+RUN curl -fsSL "https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_${FLUTTER_VERSION}-stable.tar.xz" \
+      -o /tmp/flutter.tar.xz && \
+    tar xf /tmp/flutter.tar.xz -C /opt && \
+    rm /tmp/flutter.tar.xz
+
+RUN git config --global --add safe.directory "${FLUTTER_HOME}"
+
+# Configure for Linux desktop only
+RUN flutter config --enable-linux-desktop \
+    --no-enable-web \
+    --no-enable-android \
+    --no-enable-ios \
+    --no-enable-macos-desktop \
+    --no-enable-windows-desktop
+
+# Precache only Linux artifacts
+RUN flutter precache --linux \
+    --no-web \
+    --no-android \
+    --no-ios \
+    --no-macos \
+    --no-windows \
+    --no-fuchsia \
+    --no-universal
+
+RUN flutter doctor -v
+
+WORKDIR /app