Updates the source image to 20.04, PHP 8 and MySQL 8

In addition, the new source image is not a "latest" image, which
is a more unstable tag.

Dockerfile

-FROM mattrayner/lamp:latest-1604
+FROM mattrayner/lamp:0.8.0-2004-php8
 
 ENV PHP_DISPLAY_ERRORS Off
 ADD run.sh /run.sh

README.md

 # LAMP (Linux/Apache/MySQL/PHP)
-This LAMP docker image is based on `mattrayner/lamp:latest-1604` image.
+This LAMP docker image is based on `mattrayner/lamp:0.8.0-2004-php8` image.
 You have more detailed information at https://hub.docker.com/r/mattrayner/lamp/
 
+This image has the following versions of the LAMP stack:
+- Linux: Ubuntu 20.04
+- Apache: 2.4.41
+- MySQL: 8.0.26
+- PHP: 8.0.10
+
+
 # Build the image
 First, you have to build the docker image. This process has to be done only once.
 ```