feat: support python 3.8

Update to the Python 3.8.0 image, and also switch to the alpine base so
that downloads of this action are faster.

Fixes #2

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.7.4-stretch
+FROM python:3.8.0-alpine3.10
 
 RUN pip install poetry==0.12.17