fix(docker): add user creation and permissions in Dockerfile

Co-authored-by: Copilot <copilot@github.com>

Dockerfile

@@ -2,6 +2,8 @@ FROM python:3.13.12-slim
 LABEL maintainer="Luke Tainton <luke@tainton.uk>"
 USER root
 
+RUN useradd -r -s /sbin/nologin -M user
+
 ENV PYTHONPATH="/run:/usr/local/lib/python3.13/lib-dynload:/usr/local/lib/python3.13/site-packages:/usr/local/lib/python3.13"
 ENV UV_PROJECT_ENVIRONMENT="/usr/local/"
 
@@ -24,3 +26,6 @@ ARG version="dev"
 ENV APP_VERSION=$version
 
 COPY app /run/app
+
+RUN chown -R user:user /run
+USER user