Set PYTHONIOENCODING=UTF-8 in web container Dockerfile

uWSGI's wsgi.errors stream inherits the default ASCII encoding from
the base image, causing UnicodeEncodeError when logging messages
contain non-ASCII characters (e.g. the Unicode welcome banner).

Author: Nils Bars

webapp/Dockerfile

@@ -1,5 +1,7 @@
 FROM python:3.13.1-bookworm
 
+ENV PYTHONIOENCODING=UTF-8
+
 ARG DOCKER_GROUP_ID
 
 # Install dependencies