Fix Docker container merge

While the Docker daemon had little trouble combining the two images on WSL, it wasn't able to do so in other environments. This approach is a little more universal.

Dockerfile

@@ -1,11 +1,13 @@
 FROM openjdk:slim
-COPY --from=python:3 / /
+
+# Install Python
+RUN apt-get update && apt-get install -y python3 python3-pip
 
 WORKDIR /usr/src/app
 
 # Prepare Minecraft - Matrix bridge
 COPY requirements.txt ./
-RUN pip install --no-cache-dir -r requirements.txt
+RUN pip3 install --no-cache-dir -r requirements.txt
 
 COPY src/ ./src/
 COPY main.py ./
@@ -13,4 +15,4 @@ COPY main.py ./
 # Buffer Python's stdout for debugging during runtime
 ENV PYTHONUNBUFFERED=1
 
-CMD ["python", "main.py"]
+CMD ["python3", "main.py"]