Use python:3.9-slim and change to a multi-stage container to minimize size

Dockerfile

@@ -1,19 +1,62 @@
-FROM archlinux
+# We need gcc and build-essential to install our requirements but we 
+# don't need them when run the bot so we can selectively copy artifacts
+# from this stage (compile-image) to second one (runtime-image), leaving 
+# behind everything we don't need in the final build. 
+FROM python:3.9-slim AS compile-image
 
-# Update Arch and install gcc, python and pip and remove cache
+# We don't want apt-get to interact with us,
-RUN pacman -Syu --noconfirm && pacman --noconfirm -S gcc python python-pip git && yes | pacman -Scc
+# and we want the default answers to be used for all questions.
+# Is it also completely silent and unobtrusive.
+ARG DEBIAN_FRONTEND=noninteractive
 
-# Copy requirements for the bot and install them
+# Update packages and install needed packages to build our requirements.
-COPY requirements.txt /tmp/
+RUN apt-get update && \
-RUN pip install --disable-pip-version-check --no-cache-dir --requirement /tmp/requirements.txt
+    apt-get install -y --no-install-recommends build-essential gcc
 
-# Create user
+# Create new virtual environment in /opt/venv and change to it.
+ENV VIRTUAL_ENV=/opt/venv
+RUN python3 -m venv $VIRTUAL_ENV
+ENV PATH="$VIRTUAL_ENV/bin:$PATH"
+
+# Copy and install requirements.
+COPY requirements.txt .
+RUN pip install --disable-pip-version-check --no-cache-dir --requirement requirements.txt
+
+# Change to our second stage. This is the one that will run the bot.
+FROM python:3.9-slim AS runtime-image
+
+# Copy Python dependencies from our build image.
+COPY --from=compile-image /opt/venv /opt/venv
+
+# Create user so we don't run as root.
 RUN useradd --create-home botuser
-WORKDIR /home/botuser
+RUN chown -R botuser:botuser /home/botuser && chmod -R 755 /home/botuser
 USER botuser
 
-RUN mkdir -p ~/data
+# Change directory to where we will run the bot.
+WORKDIR /home/botuser
 
-# Copy bot and run
+# Create directory for the sqlite database. 
+# You need to share this directory with the computer running
+# the container to save the data.
+RUN mkdir -p /home/botuser/data
+
+# Copy our Python bot to our home directory.
 COPY main.py .
-CMD ["python", "./main.py"]
+
+# Don't generate byte code (.pyc-files). 
+# These are only needed if we run the python-files several times.
+# Docker doesn't keep the data between runs so this adds nothing.
+ENV PYTHONDONTWRITEBYTECODE 1
+
+# Force the stdout and stderr streams to be unbuffered. 
+# Will allow log messages to be immediately dumped instead of being buffered. 
+# This is useful when the bot crashes before writing messages stuck in the buffer.
+# Has a minor performance loss. We don't have many log messages so probably makes zero difference.
+ENV PYTHONUNBUFFERED 1
+
+# Use our virtual environment that we created in the other stage.
+ENV PATH="/opt/venv/bin:$PATH"
+
+# Run bot.
+CMD [ "python", "./main.py" ]