initializers: check that env vars declared in env.example are present

If an environment variable is declared in `config/env.example`, but not
present in the actual environment, the server initialization will raise
an exception.

Empty strings are allowed (because some values are relevant only in
development or production).

config/initializers/env_vars.rb

@@ -0,0 +1,13 @@
+# Ensure that the environment variables defined in the reference env vars file
+# are present in the execution environment.
+#
+# This protects against an out-to-date environment leading to runtime errors.
+
+if ENV['RAILS_ENV'] != 'test' && File.basename($0) != 'rake'
+  reference_env_file = File.join('config', 'env.example')
+  Dotenv::Environment.new(Rails.root.join(reference_env_file)).each do |key, value|
+    if !ENV.key?(key.to_s)
+      raise "Configuration error: `#{key}` is not present in the process’ environment variables (declared in `#{reference_env_file}`)"
+    end
+  end
+end