Flutter give container rounded border

You can use like this. If you want border for all the corners you can use like bellow.

Container(
  decoration: BoxDecoration(
    color: Colors.white,
    borderRadius: BorderRadius.all(
      Radius.circular(12.0),
    ),
  ),
),

If you want rounded borders for selected sides only you can use like below.

Container(
  decoration: BoxDecoration(
    color: Colors.white,
    borderRadius: BorderRadius.only(
      topLeft: Radius.circular(10),
      topRight: Radius.circular(10),
    ),
  ),
  child: Text('Text'),
),

To make it completely circle:

Container(
      decoration: BoxDecoration(
        shape: BoxShape.circle,
      ),
    )

To make it a circle at selected spots:

Container(
      decoration: BoxDecoration(
        borderRadius: BorderRadius.only(
            topRight: Radius.circular(40.0),
            bottomRight: Radius.circular(40.0),
            topLeft: Radius.circular(40.0),
            bottomLeft: Radius.circular(40.0)),
      ),
      child: Text("hello"),
    ),

or

  Container(
      decoration: BoxDecoration(
        borderRadius: BorderRadius.all(Radius.circular(20)),
      ),
      child: ...
    )

Try using the property borderRadius from BoxDecoration

Something like

Container(
  decoration: BoxDecoration(
    border: Border.all(
      color: Colors.red[500],
    ),
    borderRadius: BorderRadius.all(Radius.circular(20))
  ),
  child: ...
)