Run container as non-root user; polish Dockerfile

Dockerfile

@@ -1,6 +1,8 @@
 FROM python:3.12-slim
 
-RUN apt-get update && apt-get install -y --no-install-recommends     ffmpeg     && rm -rf /var/lib/apt/lists/*
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    ffmpeg \
+    && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app
 
@@ -9,6 +11,12 @@ RUN pip install --no-cache-dir -r requirements.txt
 
 COPY . .
 
+RUN mkdir -p /app/data /app/uploads /app/outputs && \
+    groupadd -r appuser && useradd -r -g appuser appuser && \
+    chown -R appuser:appuser /app
+
+USER appuser
+
 EXPOSE 5000
 
 ENV PYTHONDONTWRITEBYTECODE=1