If you get this on windows in VS code click on the blue 'open settings' button. Near the bottom of the screen this opens there's a link to enter the settings, click this to open the settings.json file. Edit the file to look like this:
I don't know if this helps but I ran into the same problem with the extension PHP Intelephense in Visual Studio Code. What I did is read the instructions so the extension could work properly. So I disabled PHP Language Features by searching "@builtin PHP" in the extensions. Also, other (3rd party) PHP extensions that provide similar functionality should also be disabled for best results. Finally, I added glob patterns for non-standard PHP file extensions to the files.associations setting. For example: "files.associations": { "*.module": "PHP" }
Doing this made the error go away. I'm new to PHP too, let me know if this is the correct way of solving this problem with PHP Intelephense.
Regarding the docker-specific revision of the question, I found this thread in the VS Code PHP Intellisense git repo issue that explains how to create a wrapper script. It will involve mapping volumes, though, so take that into account.