add instructions to use a local package directory

Author: seebi

docs/develop/python-plugins/setup/index.md

@@ -74,7 +74,7 @@ This section describes which backend components are needed on the DataIntegratio
 ### Package Index Locations
 
 The basic setup allows for installation of packages from the [pypi.org](https://pypi.org/search/?q=%22cmem-plugin-%22) python package index, maintained by the [Python Software Foundation](https://www.python.org/psf-landing/).
-In order to change the index server, from where you can install python packages, you can use the following environment variables:
+In order to change the remote index server, from where you can install python packages, you need to set the following environment variables in the data integration container:
 
 -   `PIP_INDEX_URL` - Base URL of the default python package index Base URL. This should point to a repository which is compliant with [PEP 503 (the simple repository API)](https://peps.python.org/pep-0503/). If this variable is not set, the [official Python Package Index](https://pypi.python.org/simple) is used.
     -   Example Value: `https://pypi.eccenca.com/simple` (the eccenca Python Package Index holds only published Corporate Memory Python Plugins and respective dependencies)
@@ -87,6 +87,39 @@ In order to change the index server, from where you can install python packages,
 For individual needs, you can use additional environment variables known by `pip` (`PIP_TRUSTED_HOST`, `PIP_CERT`, ...).
 Please have a look at the [pip documentation](https://pip.pypa.io/en/stable/topics/configuration/#environment-variables).
 
+### Local Packages only
+
+In cases, where you have limited or disabled network capabilities to the internet, you can disable package retrieval and provide the packages in a local directory.
+To do so, you need to set the following environment variables in the data integration container:
+
+-   `PIP_NO_INDEX` - set the value as `true` to disable the package retrieval completely.
+-   `PIP_FIND_LINKS` - set to a container internal directory, where the packages and its dependencies will be provided.
+    -   Example Value: `/data/downloaded-packages`
+
+This setup will allow installation of packages and its dependencies ONLY from the given directory.
+
+As a next step, you need to provide the needed packages in this directory.
+To do so, use the [`pip download`](https://pip.pypa.io/en/stable/cli/pip_download/) command and copy or mount the downloaded files in your container.
+
+??? note "Example shell session showing the usage of `pip download`"
+
+    ```
+    $ cat requirements.txt
+    cmem-plugin-validation
+    cmem-plugin-graphql
+    cmem-plugin-kafka
+    cmem-plugin-yaml
+
+    $ pip download --only-binary=:all: -r requirements.txt --platform manylinux2014_x86_64
+    Collecting cmem-plugin-validation (from -r requirements.txt (line 1))
+    ...
+    Saved ./six-1.16.0-py2.py3-none-any.whl
+    Successfully downloaded cmem-plugin-validation ...
+    ```
+
+    Please note the `--platform` identifier. This is only needed in case you prepare the downloaded package directory
+    on a machine with a different platform compared to you server (e.g. on a Mac).
+
 ### Package Path
 
 The basic setup provides a `/data` directory inside of the DataIntegration container, where all changed files are managed in subdirectories.