Added small doc page for DOCKER_INDEX_URL variable

docs/sources/index/variable.rst

@@ -0,0 +1,23 @@
+=================================
+Docker Index Environment Variable
+=================================
+
+Variable
+--------
+
+.. code-block:: sh
+
+    DOCKER_INDEX_URL
+
+Setting this environment variable on the docker server will change the URL docker index.
+This address is used in commands such as ``docker login``, ``docker push`` and ``docker pull``.
+The docker daemon doesn't need to be restarted for this parameter to take effect.
+
+Example
+-------
+
+.. code-block:: sh
+
+    docker -d &
+    export DOCKER_INDEX_URL="https://index.docker.io"
+