Add "git" in the container to accept "gh" syntax

[lmazuel]

Currently:

Traceback (most recent call last):
  File "/usr/bin/cookiecutter", line 11, in <module>
    sys.exit(main())
  File "/usr/lib/python2.7/site-packages/click/core.py", line 716, in __call__
    return self.main(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/click/core.py", line 696, in main
    rv = self.invoke(ctx)
  File "/usr/lib/python2.7/site-packages/click/core.py", line 889, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/lib/python2.7/site-packages/click/core.py", line 534, in invoke
    return callback(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/cookiecutter/cli.py", line 106, in main
    config_file=user_config
  File "/usr/lib/python2.7/site-packages/cookiecutter/main.py", line 107, in cookiecutter
    no_input=no_input
  File "/usr/lib/python2.7/site-packages/cookiecutter/vcs.py", line 104, in clone
    raise VCSNotInstalled(msg)
cookiecutter.exceptions.VCSNotInstalled: 'git' is not installed.