Use image default instead of `/bin/sh`

https://github.com/Eugeny/tabby-docker/blob/aaf91eebd326e87c600445f025d964db51cd5793/src/session.ts#L20

This always sets the default terminal to sh instead of what the image default is, such as bash or zsh. Having to set this manually for each seems odd.

Start at src/session.ts:20 and trace how the terminal command is selected when a Docker image is opened. Confirm how the image default command is exposed, then change the selection so the image default is used instead of always choosing /bin/sh; done means images default to their own shell without manual configuration.

Written by the indexing model from the issue text.