Bump bundler from 2.2.20 to 2.2.26

https://github.com/rubygems/rubygems/blob/master/bundler/CHANGELOG.md#2225-july-30-2021

Something we _might_ get some support questions on is:
rubygems/rubygems#4647 but overall I think
that's a very good change! The GEM sources not being split has bitten
our users in the past, so happy to see that change.

Dockerfile

@@ -76,7 +76,7 @@ RUN apt-add-repository ppa:brightbox/ruby-ng \
   && apt-get install -y --no-install-recommends ruby2.7 ruby2.7-dev \
   && gem update --system 3.2.20 \
   && gem install bundler -v 1.17.3 --no-document \
-  && gem install bundler -v 2.2.20 --no-document \
+  && gem install bundler -v 2.2.26 --no-document \
   && rm -rf /var/lib/gems/2.7.0/cache/* \
   && rm -rf /var/lib/apt/lists/*