Update Dockerfile to Use Node.js 20 and Simplify Image Setup

[Oxyrobit]

The current Dockerfile uses Node.js 16 and includes manual user/group modifications (usermod, groupmod, etc.). This setup can be simplified and modernized by upgrading to Node.js 20 and relying on the official node user already provided in the base image.

Proposed Changes:

• Upgrade from node:16.13-alpine3.13 to node:20-alpine.
• Remove manual UID/GID adjustments using shadow, usermod, etc.
• Use the built-in node user directly.
• Simplify the build and runtime steps.
• Use --omit=optional and --omit=dev to optimize npm ci installs.
• Improve Dockerfile readability and maintainability.

Benefits:

• Smaller, more secure image.
• Updated to a maintained Node.js version (v20 LTS).
• Fewer layers and dependencies to manage.
• Easier to maintain and understand.

Affected File:

• Dockerfile