chore(docker): Install SkiaSharp native dependencies for captcha generation

- Add native library dependencies (libfontconfig1, libfreetype6) required by SkiaSharp
- Install dependencies in final stage before application setup
- Clean up apt cache to reduce image size
- Ensure captcha generation functionality works in containerized environment

server/MiAssessment/src/MiAssessment.Admin/Dockerfile

@@ -20,6 +20,13 @@ RUN dotnet publish "src/MiAssessment.Admin/MiAssessment.Admin.csproj" -c Release
 
 # ==================== 最终运行阶段 ====================
 FROM 192.168.195.25:19900/library/dotnet/aspnet:10.0-preview AS final
+
+# 安装 SkiaSharp 验证码生成所需的原生依赖
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libfontconfig1 \
+    libfreetype6 \
+    && rm -rf /var/lib/apt/lists/*
+
 WORKDIR /app
 COPY --from=build /app/publish .
 COPY --from=frontend /app/dist ./wwwroot