Slim down the size of docker image (#2032)

`python3-dev` is already included in `python:3.7-slim-buster`. So we don't need to install it again.
This helps us reduce more than 100MB.

Dockerfile

@@ -6,7 +6,6 @@ RUN mkdir -p /opt/CTFd /var/log/CTFd /var/uploads
 RUN apt-get update \
     && apt-get install -y --no-install-recommends \
         build-essential \
-        python3-dev \
         libffi-dev \
         libssl-dev \
         git \