How do I install PHP in the code-server docker container?

[randrewlove]

I installed code-server in a docker container running on my server at https://mydomain.com:8443/. I want to do some development using PHP. I installed the devsense PHP extension (this one). When I try to run or debug a simple PHP file in code-server, a new tab opens at URL https://mydomain.com:8443/proxy/8000/ that only shows the text connect ECONNREFUSED 0.0.0.0:8000 and back in code-server a popup message in the bottom right mentions that the php binary could not be found or executed and to make sure PHP is installed.

My server has PHP installed, but my current understanding is that I need it installed inside the code-server docker container. I went to the terminal from within code-server and tried to execute sudo apt install php in an attempt to install it inside the container, but this was the result:

abc@servername:/config/workspace/php_sandbox# sudo apt install php
[sudo] password for abc: 
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
E: Unable to locate package php

I tried searching to find any PHP installation steps specific to code-server, but I could not find anything. How is this done?