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

Currently:

``` console
Traceback (most recent call last):
File "/usr/bin/cookiecutter", line 11, in
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.
```

Start by inspecting the container build configuration and reproduce the shown cookiecutter command with a GitHub-style repository reference. Confirm how the image exposes version-control tools, then verify that the command no longer raises VCSNotInstalled and accepts the requested syntax.

Written by the indexing model from the issue text.