docs(readme): document file:// mirrors as a local download source (#405)

A local directory laid out like nodejs.org already works as a mirror, but
only https mirrors were documented. Add an example, mention it in the
--mirror description and cover it with a test that lists versions from a
local index.json.

Closes #193

Author: ekalinin

CHANGES

@@ -24,6 +24,9 @@ Version [unreleased]
   `--source` that is what lets a repeated `--force` build reuse the
   downloaded source tree
   `#205 <https://github.com/ekalinin/nodeenv/issues/205>`_
+- Documented that `--mirror` takes a `file://` URL, so a local directory can
+  serve as the download source
+  `#193 <https://github.com/ekalinin/nodeenv/issues/193>`_
 
 Version 1.3.1
 -------------

README.rst

@@ -141,6 +141,12 @@ Install node.js from a mirror::
 
     $ nodeenv --node=10.19.0 --mirror=https://npm.taobao.org/mirrors/node
 
+A local directory works as a mirror too, if it repeats the layout of
+nodejs.org: packages in ``v<version>/`` and, to resolve ``latest``, ``lts``
+or a version range, an ``index.json`` next to them::
+
+    $ nodeenv --node=22.14.0 --mirror=file:///srv/node-mirror env-22
+
 Install the highest node.js release matching a version range::
 
     $ nodeenv --node=22 env-22
@@ -307,7 +313,8 @@ Installation options
     Install node.js from the source (Unix only).
 
 ``--mirror=URL``
-    Set mirror server of nodejs.org to download from.
+    Set mirror server of nodejs.org to download from. A ``file://`` URL
+    points nodeenv at a local directory instead of a server.
 
 ``-c, --clean-src``
     Remove "src" directory after installation. This is the default.

tests/nodeenv_test.py

@@ -8,6 +8,7 @@
     from shlex import quote as _quote
 import io
 import os.path
+import pathlib
 import subprocess
 import sys
 import sysconfig
@@ -309,6 +310,20 @@ def rewind(_):
                 mock_logger.assert_called()
 
 
+def test_mirror_option_local_directory(tmpdir):
+    """A local directory is a valid mirror, see #193"""
+    tmpdir.join('index.json').write(
+        '[{"version": "v99.0.0", "date": "2026-01-01", "lts": false,'
+        ' "files": ["linux-x64", "linux-x64-musl", "linux-riscv64"]}]')
+    mirror = pathlib.Path(str(tmpdir)).as_uri()
+    argv = [__file__, '--list', '--mirror=' + mirror]
+    with mock.patch.object(sys, 'argv', argv), \
+            mock.patch.object(nodeenv.logger, 'info') as mock_logger:
+        nodeenv.src_base_url = None
+        nodeenv.main()
+        mock_logger.assert_called_with('99.0.0')
+
+
 @pytest.mark.usefixtures('mock_index_json', 'mock_host_platform')
 def test_get_latest_node_version():
     assert nodeenv.get_last_stable_node_version() == '13.5.0'