Use a pre-built Docker image base

[jgehrcke]

The Docker image build is rather heavy, and executed on every single GH actions run.

With this change, the Dockerfile in this repo uses a new image base: https://hub.docker.com/r/jgehrcke/github-repo-stats-base

This includes

• the Chrome installation and
• the heavy Python dependencies

Now we're not following python:3.8-slim-buster as a moving target anymore. This is often a red flag in security-critical environments. But not so much here. It's fine when the base gets a little out of date with the upstream Linux distro every now and then.

This approach / patch / PR is motivated by #24 and also my own observations where the Docker image build sometimes fails as of instabilities.

[jgehrcke]

$ make base-image
docker build -f base.Dockerfile . -t jgehrcke/github-repo-stats-base:9dec0ebaf
Sending build context to Docker daemon  4.146MB
Step 1/7 : FROM python:3.8-slim-buster
 ---> 4728acd2148c
Step 2/7 : RUN apt-get update && apt-get install -y -q --no-install-recommends     gnupg curl git jq moreutils ca-certificates unzip less tree pandoc     && rm -rf /var/lib/apt/lists/*
 ---> Using cache
 ---> fa3d915db0e1
Step 3/7 : RUN curl -sS -o - https://dl-ssl.google.com/linux/linux_signing_key.pub | apt-key add
 ---> Using cache
 ---> 44189d2df2be
Step 4/7 : RUN echo "deb [arch=amd64]  http://dl.google.com/linux/chrome/deb/ stable main" >> /etc/apt/sources.list.d/google-chrome.list
 ---> Using cache
 ---> 8722f8d3a56b
Step 5/7 : RUN apt-get -y update
 ---> Using cache
 ---> dbd46e4c70a6
Step 6/7 : RUN apt-get -y install google-chrome-stable
 ---> Using cache
 ---> f4f5e71762d9
Step 7/7 : RUN pip install pandas==1.3.2 PyGitHub==1.54.1 pytz retrying     selenium==3.141.0 webdriver_manager carbonplan[styles] altair==4.1.0
 ---> Using cache
 ---> 6d92e846be8b
Successfully built 6d92e846be8b
Successfully tagged jgehrcke/github-repo-stats-base:9dec0ebaf

$ make base-image-push
docker push jgehrcke/github-repo-stats-base:9dec0ebaf
The push refers to repository [docker.io/jgehrcke/github-repo-stats-base]
93bed792c1b9: Preparing 
93bed792c1b9: Layer already exists 
f230131547d3: Layer already exists 
d1e4e5062bf5: Layer already exists 
d1bf8f52195f: Layer already exists 
761903b33e48: Layer already exists 
6858b25d5a73: Layer already exists 
de9b19e6cd49: Layer already exists 
4e74918601aa: Layer already exists 
55ad5d8b8f0f: Layer already exists 
d000633a5681: Layer already exists 
9dec0ebaf: digest: sha256:d8dce9dfcf6ffd94a77a3a34d7540094b4a426b2746b80fa07f1e2d383c91279 size: 2635