[Suggestion] ✨ Adjust container and add xs size

[k-utsumi]

Try and Suggestion PR for below issue for v5.
#30969

v4 .container, .container-[sm ~ xl] sample
https://jsfiddle.net/p2s1f9zn/

I don't understand the difference in behavior,
so I tried changing to a container class with different max-width.

Also, add the xs width to be used on the login screen, etc.

[mdo]

No plans to change these or add another breakpoint at xs—we consider the default classes our "xs" tier.

Here's the container classes in effect. Basically they are 100% wide until a breakpoint.