Update dependencies - Githubissues

This PR updates some gems to fix found vulnerabilities:

update Ruby to 2.7.5 (due EOL for Ruby 2.6 next month)
bump Rails to 6.0.4.6
update brakeman to avoid false positive warning about loofah version (https://github.com/presidentbeef/brakeman/issues/1603)
update Dockerfile (to fetch Ruby version from .ruby-version during bundle install)
update puma and sidekiq gems to address security issues

fs / rails-base-graphql-api