Revert "Renamed user from app to modmail"

This reverts commit 319000b.

Dockerfile

@@ -3,18 +3,17 @@ FROM python:3.10 as py
 FROM py as build
 
 RUN apt update && apt install -y g++ git
-
 COPY requirements.txt /
 RUN pip install --prefix=/inst -U -r /requirements.txt
 
 FROM py
 
-COPY --from=build /inst /usr/local
-
 ENV USING_DOCKER yes
-RUN useradd --system --no-create-home modmail
-USER modmail
+COPY --from=build /inst /usr/local
 
 WORKDIR /modmailbot
 CMD ["python", "bot.py"]
-COPY --chown=modmail:modmail . /modmailbot
+COPY . /modmailbot
+RUN adduser --disabled-password --gecos '' app && \
+  chown -R app /modmailbot
+USER app