Pin Dockerfile Python version explicitly to a release tag

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:latest
+FROM python:3.9
 
 WORKDIR "/root/Bulk Downloader for Reddit"
 COPY ./requirements.txt ./
@@ -6,4 +6,4 @@ RUN ["pip", "install", "-r", "requirements.txt"]
 EXPOSE 8080
 EXPOSE 7634
 
-CMD ["python", "script.py", "-d", "downloads"]
+CMD ["python", "script.py", "-d", "downloads"]