It is quite unfortunate that this discussion was marked as one that has an answer: the workaround of installing extensions into "system" dir /usr/lib/code-server/lib/vscode/extensions/ is that those extensions then get treated as system ones: they aren't listed as installed extensions, yet they are running and can't be disabled.

I am trying to prepare a docker image with code-server for a group of engineers and we would like to have some set of extensions to be-preinstalled by default, but since every engineer mounts their own directory to ${HOME} the default location for user-installed extensions ~/.local/share/code-server/extensions/ thus gets overwritten by their mount and they won't see the pre-installed extensions.
Since I would like to make some extensions preinstalled, but let user disable them if they wish so - I can't use /usr/lib/code-server/lib/vscode/extensions/ as install dir.