Support to open a repository into a volume

[egamma]

Support that the user can define a git repository containing a 'devcontainer.json' file. The repository is cloned into a volume which is then opened in a container.

Cloning a repository into a volume provides the following benefits:

• improved performance on Windows and OS X in particular for larger repositories (faster than a bind mount).
• it does not require that the user has to configure Shared Drives on Windows.

Since the code is only available in the volume and the folder is not bind mounted on the host operating system, the files can only be accessed inside the container.