Another suggestion I would make is to decouple the Linux flavor from the Ruby version passed into the build context. For example, in your Dockerfile, FROM should read:
ARG RUBY_VERSION
FROM ruby:${RUBY_VERSION}-alpine
# …
Then in your docker-compose.yml file, pass only the Ruby version:
service:
web:
build:
args:
RUBY_VERSION: 2.6.8
The reason for doing this is the Dockerfile uses Alpine-specific commands for installing packages (apk instead of apt). If someone passes in "2.6.8-buster" then they've broken the Dockerfile and the build will fail.