How to make border on FloatingActionButton?

my problem is i want to do border around my floatingactionbutton. I tried to put it into container and do like this, but it’s not what i want.

        decoration: BoxDecoration(
            border: Border.all(
                color: Colors.brown, width: 5, style: BorderStyle.solid)),
        margin: const EdgeInsets.fromLTRB(0, 0, 0, 55),
        width: 80,
        height: 80,
        child: FloatingActionButton(
          focusColor: Colors.white54,
          backgroundColor: Colors.white,
          onPressed: () {},
          child: const Icon(
            Icons.add,
            color: Colors.black,
            size: 50,
          ),
        ),
      ),

I have this:

I want this:

>Solution :

just use shape: BoxShape.circle,

Container(
      decoration: BoxDecoration(
        shape: BoxShape.circle,
          border: Border.all(
              color: Colors.brown, width: 5, style: BorderStyle.solid)),
      margin: const EdgeInsets.fromLTRB(0, 0, 0, 55),
      width: 80,
      height: 80,
      child: FloatingActionButton(
        focusColor: Colors.white54,
        backgroundColor: Colors.white,
        onPressed: () {},
        child: const Icon(
          Icons.add,
          color: Colors.black,
          size: 50,
        ),
      ),

    )

output: