Depend on a specific version of Node.js

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

.github/workflows/tests.yml

@@ -40,7 +40,9 @@ jobs:
         with:
           bundler-cache: true
       - name: Setup NPM
-        uses: actions/setup-node@v1
+        uses: actions/setup-node@v3
+        with:
+          node-version-file: ".node-version"
       - name: Copy secrets and database files
         run: for i in config/*.example; do cp "$i" "${i/.example}"; done
       - name: Setup database