Add ZUUL_CACHE_DIR to zuul-cloner
Set the default cache directory using the ZUUL_CACHE_DIR variable so
users don't have to pass it explicitly every time.
This change does not set up --cache-dir like the other environment
variable-based options with a --zuul prefix because I did not want to
break backwards compatibility by changing the option name.
Change-Id: I1dab9dadda3a9ccf028561b09fdd78929d1dbfe7
diff --git a/doc/source/cloner.rst b/doc/source/cloner.rst
index 2ddf0b5..ccb04b6 100644
--- a/doc/source/cloner.rst
+++ b/doc/source/cloner.rst
@@ -87,3 +87,7 @@
The URL of origin remote of the resulting clone will be reset to use
the ``git_base_url`` and then the remote will be updated so that the
repository has all the information in the upstream repository.
+
+The default for ``--cache-dir`` is taken from the environment variable
+``ZUUL_CACHE_DIR``. A value provided explicitly on the command line
+overrides the environment variable setting.