Add --docker-image flag to override default client Docker images

[ch4r10t33r]

Add --docker-image flag to override default client Docker images

Summary

This PR adds support for the --docker-image command-line parameter to spin-node.sh, allowing users to override the default Docker image for client nodes without modifying tracked files in client-cmds/*-cmd.sh. This addresses issue #83.

Problem

Previously, updating client Docker images or flags required modifying client-cmds/*-cmd.sh files, which:

• Created uncommitted changes in the main branch
• Required a new PR for every flag/image change
• Made it difficult to test custom builds or different image tags locally

Solution

Added a new --docker-image <image[:tag]> parameter that:

• Overrides the default Docker image for all nodes when specified
• Works with any client (zeam, ream, qlean, lantern)
• Only affects Docker mode (binary mode is unchanged)
• Replaces the image name in the node_docker command before execution
• Allows local customization without modifying tracked files