After some research, I found out some solution, and thanks to @Günter Zöchbauer,
I used column instead of Container and
set the property to column CrossAxisAlignment.stretch to Fill match parent of Button
new Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: <Widget>[
new RaisedButton(
child: new Text(
"Submit",
style: new TextStyle(
color: Colors.white,
)
),
colorBrightness: Brightness.dark,
onPressed: () {
_loginAttempt(context);
},
color: Colors.blue,
),
],
),