容器的概念，是用来包含其它容器(container)和项目(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.
给view（在小程序中用view，h5里用div）的display属性设置为flex或inline-flex，这个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.
设置为display:flex时，是块级弹性容器；设置为display:inline-flex时，是行内级弹性容器。
