Fix bin/dev

- accidentally overwritten with a newer version that doesn't
  use foreman when updating to Rails 8. Doesn't affect
  govuk-docker users, but causes a surprise port change for
  people using it outside of govuk-docker.

bin/dev

@@ -1,2 +1,8 @@
-#!/usr/bin/env ruby
-exec "./bin/rails", "server", *ARGV
+#!/usr/bin/env sh
+
+if ! gem list foreman -i --silent; then
+  echo "Installing foreman..."
+  gem install foreman
+fi
+
+exec foreman start -f Procfile.dev "$@"