[twitter-bootstrap] Bootstrap: how do I change the width of the container?

For bootstrap 4 if you are using Sass here is the variable to edit

// Grid containers
//
// Define the maximum width of `.container` for different screen sizes.

$container-max-widths: (
  sm: 540px,
  md: 720px,
  lg: 960px,
  xl: 1140px
) !default;

To override this variable I declared $container-max-widths without the !default in my .sass file before importing bootstrap.

Note : I only needed to change the xl value so I didn't care to think about breakpoints.