rubygems-update requires ruby 3, breaking docker image creation

[hrdl-github]

https://rubygems.org/gems/rubygems-update/versions/3.5.0, which was released on December 15, requires ruby >= 3.0.0, which is higher than ruby 2.7. This prevents the Docker image from being built:

$ docker-compose build
...
 => ERROR [panoptes  7/10] RUN gem update --system                                                              12.1s 
------                                                                                                                
 > [panoptes  7/10] RUN gem update --system:                                                                          
12.02 ERROR:  Error installing rubygems-update:                                                                       
12.02 	There are no versions of rubygems-update (= 3.5.1) compatible with your Ruby & RubyGems
12.02 	rubygems-update requires Ruby version >= 3.0.0. The current ruby version is 2.7.8.225.
12.02 ERROR:  While executing gem ... (NoMethodError)
12.02     undefined method `version' for nil:NilClass
12.09 Updating rubygems-update
------
failed to solve: process "/bin/sh -c gem update --system" did not complete successfully: exit code: 1