How do I set the cookie lifetime?
For the session cookie options D8 uses container parameters instead of settings. Create a services.yml file in the same folder as settings.php. The default values are in default.services.yml. You can copy this file to services.yml and modify it:

/sites/default/services.yml:

parameters:
  session.storage.options:
    # Default ini options for sessions.
    #
    # Some distributions of Linux (most notably Debian) ship their PHP
    # installations with garbage collection (gc) disabled. Since Drupal depends
    # on PHP's garbage collection for clearing sessions, ensure that garbage
    # collection occurs by using the most common settings.
    # @default 1
    gc_probability: 1
    # @default 100
    gc_divisor: 100
    #
    # Set session lifetime (in seconds), i.e. the time from the user's last
    # visit to the active session may be deleted by the session garbage
    # collector. When a session is deleted, authenticated users are logged out,
    # and the contents of the user's $_SESSION variable is discarded.
    # @default 200000
    gc_maxlifetime: 200000
    #
    # Set session cookie lifetime (in seconds), i.e. the time from the session
    # is created to the cookie expires, i.e. when the browser is expected to
    # discard the cookie. The value 0 means "until the browser is closed".
    # @default 2000000
    cookie_lifetime: 2000000