Answers for "laravel/framework[v5.8.0, ..., v5.8.38] 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