Add userid argument in Dockerfile.

Should fix #45.

Author: skytreader

Dockerfile

@@ -1,14 +1,16 @@
 FROM python:3.7-stretch
 LABEL maintainer "[email protected]"
 
+ARG userid=1000
+
 RUN apt-get update && \
     pip install pygame
 
 COPY ./requirements.txt /requirements.txt
 RUN pip install -r /requirements.txt
 COPY ./ app/
 RUN cd app/ && python setup.py install
-RUN useradd -m -U -s /bin/bash pygame
+RUN useradd -m -U -s /bin/bash -u $userid pygame
 
 USER pygame
 WORKDIR /home/pygame

README.md

@@ -15,8 +15,15 @@ commmon code patterns I find while using PyGame.
 as this relies on language features only available from there.
 
 # Development
-Aside from `.travis.yml`, the Dockerfile is provided for development. You can
-also pull the image via
+
+Aside from `.travis.yml`, the Dockerfile is provided for development. The
+Dockerfile takes an argument `userid` which should be a user id outside the
+container that has access to `/tmp/.X11-unix`. If you are in a graphical desktop
+environment, it would suffice to pass the `$UID` environment variable like so,
+
+    docker build -t pygame-objects --build-arg userid=$UID .
+
+You can also pull the image via
 
     docker run skytreader/pygame-objects:latest