rb-inotify requires Ruby version >= 2.2 (installing compass)

compass 1.0.3 depends on rb-inotify >= 0.9 and rb-inotify launched a new version on December 15, 2018 which depends on Ruby >= 2.2 (the Ruby version installed through 'apt-get install -y ruby-full' on node:6.10 is 'ruby 2.1.5p273')

# lock rb-inotify version because the next one (0.10.0) depends on ruby >= 2.2
RUN gem install --no-rdoc --no-ri rb-inotify -v 0.9.10
# -​-conservative to instruct it to use the existing rb-inotify (0.9.10) instead of resolving rb-inotify >= 0.9
RUN gem install --no-rdoc --no-ri compass -v 1.0.3 -- -​-conservative