Replace image with Ruby:2.7+

[ckenst]

GitHub doesn't allow going beyond Jekyll 3.9. Other dependencies (like activesupport) need later versions of Ruby such as 2.7. For this reason we need to create our own image or rebuild our docker-compose file to allow the use a generic ruby image and then mount and install the jekyll dependencies.

version: "2"
services:
  testingconferences:
    image: jekyll/jekyll:3.8.6
    container_name: tcorg
    ports:
      - 4000:4000 # jekyll ui
    command: sh -c "bundle install && jekyll serve"
    restart: unless-stopped
    platform: linux/amd64
    volumes:
      - ./:/srv/jekyll

[ckenst]

This was done in #794 but issue didn't get closed.