WordPress - Gutenberg -> Custom image block (render_block hook)

blocks.php

<?php
    // php library Simple HTML DOM Parser https://simplehtmldom.sourceforge.io/ (required)
    require_once __DIR__ . '/simple-html-dom.php';

    // rewrites core-image block with custom HTML
    add_filter( 'render_block', 'my_block_core_image', 10, 2 );

    function my_block_core_image( $block_content, $block ) {

        // specify image block
        if ( $block['blockName'] === 'core/image' ) {	

            // this dump of image block HTML will help you with parsing (just for example)
            var_dump( $block_content );

            // create a DOM object from a string
            $html = str_get_html( $block_content );

            // finds the first required element and saves it to variable, if not returns null
            $figure = $html->find( 'figure', 0 );
            $link = $html->find( 'a', 0 );
            $img = $html->find( 'img', 0 );
            $caption = $html->find( 'figcaption', 0 );

            // turns on output buffering
            ob_start();

                // gets attributes from specified elements
                ?>
                    <figure class="<?php echo $figure->class; ?>">
                        <?php
                            if ( $link ) {
                                echo '<a href="' . $link->href . '">';
                            }
                        ?>

                        <img class="<?php echo $img->class; ?>" src="<?php echo $img->src; ?>" alt="<?php echo esc_attr( $img->alt ); ?>">

                        <?php
                            if ( $link ) {
                                echo '</a>';
                            }

                            if ( $caption ) {
                                ?>
                                    <figcaption>
                                        <?php
                                            echo $caption->innertext;
                                        ?>
                                    </figcaption>
                                <?php
                            }
                        ?>
                    </figure>
                <?php
            // returns the content of output buffering
            return ob_get_clean();
        }

        // returns other blocks
        return $block_content;
    }