Add build script to inject env vars at runtime

[dragosgd]

Purpose

Refactor frontend build to support runtime environment variables

Changes

• Move from build-time to runtime environment variable injection
• Add build-and-serve.sh script that builds then serves the application
• Switch from multi-stage to single-stage Dockerfile for runtime builds
• Environment variables (TOKEN_ID, BACKEND_URL) now configured at deployment time
• Enables single image for multiple environments

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

CLA acceptance

By submitting the contribution I accept the terms and conditions of the
Contributor License Agreement v1.0

• link: https://developers.concordium.com/CLAs/Contributor-License-Agreement-v1.0.pdf

• I accept the above linked CLA.