Renamed user from app to modmail

- Rename the user from 'app' to 'modmail', and made it a system user instead.
- Moved user creation to a higher layer in the Dockerfile.


Signed-off-by: Taku <45324516+Taaku18@users.noreply.github.com>

Author: Taaku18

Dockerfile

@@ -3,17 +3,18 @@ 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
 
-ENV USING_DOCKER yes
 COPY --from=build /inst /usr/local
 
+ENV USING_DOCKER yes
+RUN useradd --system --no-create-home modmail
+USER modmail
+
 WORKDIR /modmailbot
 CMD ["python", "bot.py"]
-COPY . /modmailbot
-RUN adduser --disabled-password --gecos '' app && \
-  chown -R app /modmailbot
-USER app
+COPY --chown=modmail:modmail . /modmailbot