Update all the elements of JSON array nested in a JSON column (Postgres)

I have json column with data that looks like this:

CREATE TABLE foo(mycol)
AS SELECT $${
    "a": [
        { "q": 5 },
        { "q": 8 },
        { "q": 10 }
    ]
}$$::jsonb;

I'd like to map over the elements in the array and add an extra property to get something like this:

{
    a: [
        { q: 5, r: [] },
        { q: 8, r: [] },
        { q: 10, r: [] }
    ]
}

Any and all would be greatly appreciated. PostgreSQL 9.6.6