Refactor Dockerfile to not run as root

https://flask.palletsprojects.com/en/stable/deploying/gunicorn/#binding-externally

Dockerfile

@@ -1,5 +1,12 @@
+# syntax=docker/dockerfile:1
 FROM python:3.13.7-alpine
 
+ARG UID=1000
+ARG GID=1000
+
+RUN addgroup -g $GID thirteener
+RUN adduser -D -g '' -u $UID -G thirteener thirteener
+
 # Generic labels
 LABEL maintainer="Arian Mollik Wasi <arianmollik323@gmail.com>"
 LABEL version="0.3.4"
@@ -18,9 +25,13 @@ LABEL org.opencontainers.image.description="My own custom 12ft.io replacement"
 LABEL org.opencontainers.image.documentation="https://github.com/wasi-master/13ft/blob/main/README.md"
 LABEL org.opencontainers.image.licenses=MIT
 
+RUN python -m pip install --upgrade pip
+
+USER thirteener
 COPY . .
-RUN pip install -r requirements.txt
+RUN --mount=type=cache,mode=0755,target=/home/thirteener/.cache/pip \
+  pip install -U -r requirements.txt
+
 WORKDIR /app
 EXPOSE 5000
-ENTRYPOINT [ "gunicorn" ]
-CMD [  "portable:app" ]
+CMD [ "python", "-m", "gunicorn", "portable:app" ]