Install the current Bundler in Dockerfile

In `Dockerfile`, we have been specifying the exact Bundler version to
install. This is no longer necessary.

This commit updates `Dockerfile` to install the current version of
Bundler. Bundler will then take care of setting up the correct version
to use.

Ref:
- exercism/ruby#1635 (comment)

Dockerfile

@@ -5,7 +5,7 @@ RUN apk update && apk upgrade && \
 
 COPY Gemfile Gemfile.lock .
 
-RUN gem install bundler:2.4.18 && \
+RUN gem install bundler && \
     bundle config set without 'development test' && \
     bundle install