Write a function that takes the number of seconds and returns the digital format clock time as a string. Time should be counted from 00:00:00.
digital_clock(5025) ➞ "01:23:45"
# 5025 seconds is 1 hour, 23 mins, 45 secs.
digital_clock(61201) ➞ "17:00:01"
# No AM/PM. 24h format.
digital_clock(87000) ➞ "00:10:00"
# It's 00:10 next day.
seconds is always greater than or equal to 0.