hardcode bundler version to install

Installatio no longer worked; would fail with

#19 [15/21] RUN gem install bundler
#19 51.00 ERROR:  Error installing bundler:
#19 51.00 	The last version of bundler (>= 0) to support your Ruby & RubyGems was 2.4.22. Try installing it with `gem install bundler -v 2.4.22`
#19 51.00 	bundler requires Ruby version >= 3.0.0. The current ruby version is 2.7.8.225.
#19 ERROR: process "/bin/sh -c gem install bundler" did not complete successfully: exit code: 1
------
 > [15/21] RUN gem install bundler:
51.00 ERROR:  Error installing bundler:
51.00 	The last version of bundler (>= 0) to support your Ruby & RubyGems was 2.4.22. Try installing it with `gem install bundler -v 2.4.22`
51.00 	bundler requires Ruby version >= 3.0.0. The current ruby version is 2.7.8.225.
------
Dockerfile:47

Dockerfile

@@ -44,7 +44,7 @@ COPY . .
 
 ENV RUBYOPT="-KU -E utf-8:utf-8"
 ENV PATH="${PATH}:/redis-git/src/redis-cli:/redis-git/src/redis-server"
-RUN gem install bundler
+RUN gem install bundler -v 2.4.22
 RUN bundle update --bundler
 RUN bundle install
 RUN ./createdb