We're choosing version 18 because if offers support for SSL 3, just
like Ruby 3.1 does.
Note we're symlinking a .nvmrc file as well, in order to make it
compatible with NVM. While the .nvmrc and .node-version files use
different formats, they both support the syntax we're using to
define the version.
The code to install Node.js in the Dockerfile is a simplification of the
code in the Rails Dockerfile template [1].
[1] https://github.com/rails/rails/blob/04c97aec8a/railties/lib/rails/generators/rails/app/templates/Dockerfile.tt#L25
