ADD: Testnet-faucet build

[Thahara]

Purpose

Add build for creating docker release of testnet-faucet.

Changes

Added build action, the NEXT_PUBLIC_... environment variables have been added to the testnet-faucet environment in Github under the variable name TESTNET_FAUCET_NEXT_PUBLIC.

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.