http://stackoverflow.com/questions//container-fluid-vs-container
https://www.jquery-az.com/bootstrap-container-and-container-fluid-what-is-the-difference/
The .container class is the fixed width container. That does not mean it is not responsive. It is responsive; however, it is fixed based on screen size. The screen sizes include:
xs for extra small devices (used for less than 768 e.g smart phones, mobile etc.) sm for small screens (From 768 pixels and up e.g. tablets) md for the medium screen (>= 992 pixels. Desktops/Laptops) lg for large screens (>= 1200. Pixels e.g. large desktops)
The container-fluid class will take the full width of the viewport. If you use the container-fluid and resize the browser, you may notice the content inside it will adjust with every pixel to take the full available width.