Add non-root flutter user to all SDK images

Creates a dedicated flutter user and switches to it as the default.
Resolves Docker Scout compliance check for non-root default user.

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

Dockerfile.linux

@@ -52,4 +52,10 @@ RUN flutter precache --linux \
 
 RUN flutter doctor -v
 
+# Create non-root user for CI builds
+RUN groupadd -r flutter && useradd -r -g flutter -m -d /home/flutter flutter && \
+    chown -R flutter:flutter "${FLUTTER_HOME}" /home/flutter
+
 WORKDIR /app
+RUN chown flutter:flutter /app
+USER flutter