Update builder configuration in railway.json

[emidiovaleretto]

What

Removes the startCommand from railway.json, letting the Dockerfile's
CMD define how the app starts.

Why

The production deploy kept failing with:
Error: '$PORT' is not a valid port number.

The start command was defined in three places (railway.json startCommand,
Dockerfile CMD, and the Procfile), and railway.json takes priority over
the others. Its startCommand passed $PORT in a form that wasn't expanded
by a shell, so gunicorn received the literal string $PORT instead of the
port Railway injects.

The Dockerfile CMD already uses the shell form, which expands $PORT
correctly. Removing the startCommand makes the Dockerfile CMD the single
source of truth for the start command and resolves the port error.

Notes

• builder is kept as DOCKERFILE so the build still uses our Dockerfile
  (which installs gunicorn from production.txt)

How to verify

• Build Logs show a real build (pip install runs, gunicorn is installed)
• Deploy Logs no longer show the $PORT error and gunicorn starts
• The app responds at the Railway URL