custom border

import 'package:flutter/material.dart';

void main() => runApp(new MaterialApp(home: new app()));

class app extends StatefulWidget {
  @override
  _appState createState() => new _appState();
}

class _appState extends State<app> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        child: CustomPaint(
          painter: BorderPainter(),
          child: ClipPath(
            clipper: CustomClip(),
            child: Container(child: Image.network(
              "https://cdn.websites.hibu.com/c0c36335f54c47d89c0df32c0b7171e0/dms3rep/multi/mobile/Pizza%202_pGykYRpdQ1iN9d6Ohsnn-306x229.jpg",
              width: double.infinity,fit: BoxFit.fitWidth,),),
          ),
        ),
      ),
    );
  }
}

class BorderPainter extends CustomPainter{
  @override
  void paint(Canvas canvas, Size size) {
    Paint paint = Paint()
      ..style = PaintingStyle.stroke
      ..strokeWidth=10.0
      ..color = Colors.red;
    Path path = Path();
    path.lineTo(0.0, 0.0);
    path.lineTo(size.width, 0.0);
    path.lineTo(size.width, size.height * 0.8);
    path.lineTo(0.0, size.height);
    path.close();
    canvas.drawPath(path, paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true;

}
class CustomClip extends CustomClipper<Path> {
  @override
  Path getClip(Size size) {
    Path path = Path();
    path.lineTo(0.0, 0.0);
    path.lineTo(size.width, 0.0);
    path.lineTo(size.width, size.height * 0.8);
    path.lineTo(0.0, size.height);
    path.close();
    return path;
  }

  @override
  bool shouldReclip(CustomClipper<Path> oldClipper) => true;
}