fix: update docker image tp python 3.8 instead of 3.7

docker/Dockerfile

@@ -36,7 +36,7 @@
 ##
 ##   $ docker run -e TEST_ISS_KEEP_DATA=true --entrypoint=pytest spacetx/starfish -vsxk TestWithIssData
 ##
-FROM python:3.7-slim-stretch
+FROM python:3.8-slim-buster
 
 RUN useradd -m starfish
 USER starfish