Set user home_dir to app

In the previous configuration the webdev user had no home. Some tools
like pip use the home dir to store config files, so it's a good thing
to have it.

Dockerfile

@@ -4,7 +4,7 @@ ENV PYTHONDONTWRITEBYTECODE 1
 EXPOSE 8000
 CMD ["./bin/run-prod.sh"]
 
-RUN adduser --uid 1000 --disabled-password --gecos '' --no-create-home webdev
+RUN useradd --uid 1000 --no-create-home --home-dir /app webdev
 
 RUN apt-get update && \
     apt-get install -y --no-install-recommends build-essential python3 python3-dev python3-pip \