display: flex
flex-direction:
justify-content:
주축은 flex-direction에 따라 달라진다.
flex-direction
flex-direction: row | row-reverse ⇒ 교차축은 y축(세로)
flex-direction: row | row-reverse
flex-direction: row
justify-content: flex-start
justify-content: flex-end
justify-content: center