Three old homeless men had an empty can gathering competition in the park.
A sky can is a can that has nothing but sky in it. It's an empty can.
The reading for 空き isn't one you've seen before. Remember how 開く means "to open"? Well, this can is also open, because nothing is filling it. The readings are the same between the two, as well. 開く and 空く, though in this case it becomes the stem form: 空き.