```dockerfile
# Use a stable Python base image
FROM python:3.11-slim

# Build arguments for host user mapping
ARG USER_ID=1000
ARG GROUP_ID=1000

# Create non-root user
RUN groupadd -g ${GROUP_ID} infoscreen_taa \
    && useradd -u ${USER_ID} -g ${GROUP_ID} --shell /bin/bash --create-home infoscreen_taa

# Ensure user exists
RUN getent passwd infoscreen_taa

# Install locale dependencies and generate UTF-8 locale
RUN apt-get update && apt-get install -y locales \
    && sed -i 's/# de_DE.UTF-8 UTF-8/de_DE.UTF-8 UTF-8/' /etc/locale.gen \
    && locale-gen

# Set environment variables for locale
ENV LANG=de_DE.UTF-8 \
    LANGUAGE=de_DE:de \
    LC_ALL=de_DE.UTF-8

# Enable Dash debug during development
ENV DASH_DEBUG_MODE=True

# Working directory inside container
WORKDIR /app  # entspricht mount in devcontainer.json

# Copy only requirements first for efficient caching
COPY server/requirements-dev.txt ./

# Install dev dependencies under the non-root user
USER infoscreen_taa
RUN pip install --upgrade pip \
    && pip install --user -r requirements-dev.txt

# Switch back to root to copy source files and fix permissions
USER root

# Copy the server application code into /app
COPY server/ /app
RUN chown -R infoscreen_taa:infoscreen_taa /app

# Create config directory under the non-root user's home
RUN mkdir -p /home/infoscreen_taa/.config/Infoscreen-Server \
    && chown -R infoscreen_taa:infoscreen_taa /home/infoscreen_taa/.config/Infoscreen-Server

# Expose development ports
EXPOSE 8000 8050

# Use a long-running process so the container stays alive
CMD ["tail", "-f", "/dev/null"]
```