Use slim version of docker container (#34)

* Use slim version of docker container

This massively reduces the size of the final container (330mb -> 60mb)

* Install libcurl and libssl explicitly in container

They appear to be missing from `-slim` variant containers

Co-authored-by: Ben Busby <benbusby@pm.me>

Co-authored-by: Ben Busby <benbusby@pm.me>

Dockerfile

@@ -1,7 +1,7 @@
-FROM python:3.8
+FROM python:3.8-slim
 
 WORKDIR /usr/src/app
-
+RUN apt-get update && apt-get install -y build-essential libcurl4-openssl-dev libssl-dev
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt