容器的概念，是用來包含其它容器(container)和項目(item)。
flex container——flex容器
A flexbox layout is defined using the flex or inline-flex values of the display property on the parent item. This element then becomes a flex container, and each one of its children becomes a flex item.
給view（在小程序中用view，h5里用div）的display屬性設置為flex或inline-flex，這個view就成了flex容器，view里的元素就成了flex item。
A value of flex causes the element to become a block level flex container, and inline-flex an inline level flex container. These values create a flex formatting context for the element, which is similar to a block formatting context in that floats will not intrude into the container, and the margins on the container will not collapse with those of the items.
設置為display:flex時，是塊級彈性容器；設置為display:inline-flex時，是行內級彈性容器。