[Question] One devcontainer multiple projects

Hi,

First sorry if my question is obvious already answered... But it seems I can't find what I want to do. To keep it simple I detail our use case:

Basically we have several PHP projects. Some devs are on VSCode, some not.
What I try to do, is to have a prebuilt ~~container~~ image not tight to any specific project. The prebuilt ~~container~~ image is already built in a Gitlab pipeline.
I want to run one container on its own from that image and each project can be attached to on developer demand (at least for vscode peeps).

I tried to run the container with the CLI, but it asked for a workspace (which I don't have specifically, so I put a random one) and when using the "attach to container" option, the workspace is not mounted (I followed this https://code.visualstudio.com/docs/devcontainers/attach-container#_attached-container-configuration-files).
If I run it directly with a docker run command, it doesn't stay up.

It is funny, when I write this it seems just obvious, but it seems I can't make it work! So if you have any input on what I am doing wrongly or I misunderstand anything, I'll be happy.

Thanks all