Dev container improvements
- 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.