Override image maximum width in theme (Using Gutenberg editor)

add_theme_support('gutenberg', ['wide-images' => true ]) is telling gutenberg, hey this sites content area’s CSS is setup to handle wide images. The theme_support wide-images code isn’t a magic fix – instead it’s an aknowledgment from the theme developer to gutenberg, that the themes CSS’s ready for gutenberg wide content.

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If you’re just experimenting, and only want that CSS change done to pages that use gutenberg, you can use a snippet like this to check if the page is gutenberg:

add_action('body_class', function($classes){
    if (function_exists('the_gutenberg_project') && gutenberg_post_has_blocks( get_the_ID() ) )
        $classes[] = 'using-gutenberg';
    return $classes;
});

The you can better select your elements for gutenberg only, making your CSS like

body.using-gutenberg #container.container {
    /* ..wtv full width css */
}

If you’re curious about the how the content CSS is done, is in relation to Gutenberg, checkout the gutenberg-starter-theme on GitHub for reference.