`bundle install --deployment` tries to rewrite Gemfile_next.lock

[HoneyryderChuck]

Running the following command when preparing the production image fails (DEPENDENCIES_NEXT is unset btw):

> bundle install --deployment --without="development test"

#OUTPUT
Installing bootboot 0.2.2
The dependencies in your gemfile changed, but the lockfile can't be updated because frozen mode is set

You have added to the Gemfile:
* rails (~> current_version)

You have deleted from the Gemfile:
* rails (~> next_version)

Run `bundle install` elsewhere and add the updated Gemfile_next.lock to version control.


If this is a development machine, remove the Gemfile_next.lock freeze by running `bundle config set frozen false`.

I've seen this warning, but I'm running bundler 2.6.9, so I guess that shouldn't be necessary?