⚡ Use upstream coveralls Docker image, refs #9

The Docker image got implemented upstream and we're now using it, refs:
TheKevJames/coveralls-python#246

Dockerfile

@@ -1,8 +1,5 @@
-# Container image that runs your code
-FROM andremiras/coveralls:latest
+FROM thekevjames/coveralls:latest
 
-# Copies your code file from your action repository to the filesystem path `/` of the container
 COPY src/ /src/
 
-# Code file to execute when the docker container starts up (`entrypoint.py`)
 ENTRYPOINT ["/src/entrypoint.py"]