update Dockerfile to build distroless image

[borja-rivera]

This pull request updates the Dockerfile to use a distroless image for the production environment, simplifying the setup and improving security. The changes also adjust permissions and environment configurations to align with the new image.

Dockerfile updates for production:

• Switched the base image for the production environment to gcr.io/distroless/nodejs18:nonroot, replacing the previous base image (node:18-alpine). This enhances security by minimizing the attack surface of the container.
• Removed the creation of nodejs and nextjs system users and groups, as the distroless image operates without a shell or user management.
• Simplified file copying by removing chown commands, as the distroless image does not require explicit ownership changes.

Environment and runtime configuration:

• Added the CADENCE_WEB_PORT environment variable with a default value of 8088 and exposed port 8088 for the application.
• Updated the CMD instruction to directly execute server.js, removing the shell-based command setup.

Testing

These changes have been tested on a staging cluster and the application works correctly with the new setup.