To set a fixed width for your Bootstrap 3 container (instead of the default responsive behavior), you have several options:
Option 1: Override Bootstrap's Container Class (Recommended)
/* In your custom CSS file */
.container {
  width: 1200px; /* Your desired fixed width */
  max-width: none !important; /* Disables responsive behavior */
}
Option 2: Create a Custom Fixed-Width Container
<div class="container-fixed">
  <!-- Your content here -->
</div>
CSS 
.container-fixed {
  width: 1200px;
  margin-right: auto;
  margin-left: auto;
  padding-left: 15px;
  padding-right: 15px;
}