RANGER-5365:Add test users into Ranger Docker Base Image

[rameeshm]

What changes were proposed in this pull request?

This patch is to include test users in the Ranger Base image to make those users available in ranger and associated containers

[Copilot]

Pull request overview

This pull request refactors user and group creation in the Ranger Docker base image by extracting the inline shell commands into a dedicated script, and adds test users (testuser1, testuser2, testuser3) in a testgroup to support testing scenarios.

• Replaces inline RUN commands with a reusable bash script for creating users and groups
• Adds test users (testuser1, testuser2, testuser3) and testgroup to the base image
• Consolidates user creation logic into helper functions for better maintainability

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
docker/create_users_and_groups.sh	New script that creates all required users and groups including the new test users with reusable helper functions
docker/Dockerfile	Removes inline user/group creation commands and integrates the new script for user setup

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