Dev container improvements

* Add a readme
* Base the dev Docker image on the image used in CI.
* Upgrade from Bash to Zsh, with oh-my-zsh.
* Move ccache directory outside the repo, to keep the repo more clean.
* Use the real username of the current user.
* Use real path names inside the container.
* Allow using vim instead of nano.
* Enable Git clone with SSH.
* Install some nice-to-have command line utilities.
* Install a couple of nice-to-have VSCode extensions.