Revert the last commit.

There aren't Alpine 3.13 tags for the Python image yet.

Dockerfile

 # hadolint ignore=DL3006
 FROM registry.shore.co.il/cron as supersonic
 
-FROM registry.hub.docker.com/library/python:3.6-alpine3.13
+FROM registry.hub.docker.com/library/python:3.6-alpine3.12
 COPY --from=supersonic /usr/local/bin/supersonic /usr/local/bin/
 # hadolint ignore=DL3013
 RUN pip install --no-cache-dir pipenv