[PATCH v2] docs: set the GID in the .env file

[Lukas Bulwahn]

From: Pranav Annam <pranavannam at gmail.com>

The .env file needs the UID and GID for a working docker-compose build.

If you follow the current instructions, the docker build fails with a
clear error message: 'You must define GID in .env'

It is still good to update documentation to reduce the burden on new
contributors to run into this build failure first.

Signed-off-by: Pranav Annam <pranavannam at gmail.com>
[simply provide bash command in docs, reworked commit message]
Signed-off-by: Lukas Bulwahn <lukas.bulwahn at gmail.com>
---
 docs/development/installation.rst | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/docs/development/installation.rst b/docs/development/installation.rst
index 03ad67a..0a6e71c 100644
--- a/docs/development/installation.rst
+++ b/docs/development/installation.rst
@@ -25,11 +25,12 @@ configure Patchwork using Docker:
 #. Install `docker`_ and `docker-compose`_.
 
 #. Create a ``.env`` file in the root directory of the project and store your
-   ``UID`` attribute there.
+   ``UID`` and ``GID`` attribute there.
 
    .. code-block:: shell
 
       $ echo "UID=$UID" > .env
+      $ echo "GID=`id -g`" >> .env
 
 #. Build the images. This will download over 200MB from the internet:
 
-- 
2.17.1