Add Ruby version to Gemfile

Since Bundler 2.4.19, including in Ruby 3.2.3, it's possible to define
the Ruby version in the Gemfile by indicating which file contains the
version [1].

There are at least two practical cases where this is an advantage.

First, people using RVM in development will no longer accidentally run
the application using the wrong Ruby version (which, before these
changes, might happen if they switch to a branch using a different Ruby
version and forget to run `rvm use` or exit the current folder and enter
it again) because they will get an error when trying to do so.

Second, people using services like Heroku no longer need to modify the
Gemfile.

The disadvantage is that, now, every time we update the Ruby version, we
have to remember to run `bundle` so our `Gemfile.lock` gets the new
version.

[1] https://github.com/rubygems/rubygems/releases/tag/bundler-v2.4.19

Dockerfile

@@ -46,6 +46,7 @@ RUN curl -sL https://github.com/nodenv/node-build/archive/master.tar.gz | tar xz
 
 # Use the Gemfiles as Docker cache markers. Always bundle before copying app src.
 # (the src likely changed and we don't want to invalidate Docker's cache too early)
+COPY .ruby-version ./
 COPY Gemfile* ./
 RUN bundle install