Remove docker-related files.

Remove docker-related files, which don't appear to be used in
our own CI workflows.

Dockerfile

-FROM ruby:2.7
-
-ENV LC_ALL C.UTF-8
-ENV LANG en_US.UTF-8
-ENV LANGUAGE en_US.UTF-8
-
-WORKDIR /usr/src/app
-
-COPY Gemfile just-the-docs.gemspec ./
-RUN gem install bundler && bundle install
-
-EXPOSE 4000

docker-compose.yml

-version: "3.5"
-
-services:
-  jekyll:
-    build:
-      context: ./
-    ports:
-      - 4000:4000
-    volumes:
-      - .:/usr/src/app
-    stdin_open: true
-    tty: true
-    command: bundle exec jekyll serve -H 0.0.0.0 -t
-