Make master-api image configurable and enable local build in installer

[podcctv]

Motivation

• Standardize on the published master API image while still allowing local builds or overrides via an environment variable.
• Make the installer script configurable so users can specify a different master API image/tag at install time.
• Simplify starting the master stack by passing the chosen image through the installer into docker-compose commands.

Description

• Change the master-api service default image in docker-compose.yml to ghcr.io/podcctv/iperf3-master-api:latest and re-enable build: ./master-api for local builds.
• Add a new MASTER_IMAGE variable with default ghcr.io/podcctv/iperf3-master-api:latest in install_master.sh.
• Thread MASTER_IMAGE through the installer to the docker-compose build and up invocations so the image can be overridden via environment or CLI.
• Add a --master-image CLI option and update the usage/help text to document the new flag and default value.

Testing

• No automated tests were run for this change.

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Codex Task