Answers for "laravel/framework[v5.4.0, ..., v5.4.36] require ext-mbstring * -> it is missing from your system. Install or enable PHP's mbstring extension."

laravel/framework[v7.29.0, ..., 7.x-dev] require ext-mbstring * -> it is missing from your system. Install or enable PHP's mbstring extension.

Install the required mbstring extension

sudo apt-get install php-mbstring 
In Dockerfile you should add:

RUN docker-php-ext-install mbstring

phpoffice/phpspreadsheet[1.1.0, ..., 1.15.0] require ext-gd * -> it is missing from your system. Install or enable PHP's gd extension

composer require "ext-gd:*" --ignore-platform-reqs