SMO 2017 Open Round 1 Question 19

For any positive integer $n$, let $a_n$ be the units digit of the sum $1+2+3+\cdots+n$. Thus $a_1=1$, $a_2=3$, $a_3=6$, $a_4=0$. Determine $a_r$ for $r=2017^{2017}$.