Support attach to running container

[ndoschek]

Feature Description:

Add support for attaching Theia to an already running Docker container, injecting the Theia backend, and opening a remote workspace inside it.

This includes:

• List running containers and let the user pick one
• Inject the Theia backend into the container and establish a remote connection
• Determine a sensible workspace path (from container mounts, working directory, or user selection)
• Respect DOCKER_HOST for non-default Docker sockets
• Do not stop the container on disconnect
• Optionally read and apply devcontainer.json if present in the workspace (remoteUser, remoteEnv, postAttachCommand, port forwarding, extensions/settings)
• Run postConnect hooks (SSH/git config sharing, port setup)
• Show a warning before attaching: the container may execute arbitrary code, only attach to containers whose origin you trust