Offline --install-extension on host

We have a complete offline development environment. Both extensions and vscode itself are stored in an internal repository. We cannot use a devcontainer.json file due to restrictions with docker. We can however build a devcontainer and then attach to it - this is excellent thank you. However, we want the extensions already installed in the devcontainer as this is what saves people the time during projects. We cannot invoke "code --install-extension" inside the dockerfile due to this: - https://github.com/microsoft/vscode-remote-release/issues/1042 and we can't unpack the vsix as there is a special way of placing the contents in subfolders based on the manifest. Can we have a mechanism that will install extensions in the host without using devcontainer.json?

The issue names Dockerfile-based builds, host VS Code, and locally stored VSIX extensions, but no repository file or test entry point. Begin by tracing existing extension-installation paths and the constraints in linked issue #1042; done would be an offline installation mechanism that works without devcontainer.json.