Update Dockerfile for RubyGems 2.7.0+

As of RubyGems 2.7.0 (released last month), Bundler is now a default
(aka "vendored") gem. As a result we need to either accept the default
bundler gem or explicitly force our own using `gem install --force`.

I'm taking the latter approach because I believe it's less brittle for
Docker images that depend on this one. Specifically, once we've installed
*one* version of the bundler gem, subsequent `gem install bundler` calls
will work without `--force`. So, if a dependent Dockerfile includes a step
`RUN gem install bundler`, that should continue to work without error.

Also, this is the approach taken by the docker-library/ruby image.

See also: rubygems/rubygems#2058
          docker-library/ruby#165

Author: Frank Macreery

Dockerfile.erb

@@ -21,7 +21,7 @@ RUN BUILD_DIR="/tmp/ruby-build" \
  && rm -rf /var/lib/apt/lists/*
 
 RUN gem update --system
-RUN gem install bundler
+RUN gem install --force bundler
 
 ADD test /tmp/test
 RUN bats /tmp/test