feat: add CROC_PORT and CROC_PORTS env vars

[hangone]

• Add CROC_PORTS and CROC_PORT environment variables for the relay subcommand, matching the pattern used by CROC_PASS, CROC_RELAY, etc.
• Update Docker healthcheck to dynamically use these env vars instead of hardcoded port 9009
• Document the new env vars in README

Fixes #1096

[Copilot]

Pull request overview

Adds environment-variable configuration for relay ports and updates the Docker healthcheck/docs so containers remain healthy when custom relay ports are used (Fixes #1096).

Changes:

• Add CROC_PORTS / CROC_PORT env var support for the relay subcommand flags.
• Update Docker HEALTHCHECK to probe ports derived from CROC_PORTS / CROC_PORT instead of hardcoded 9009.
• Document the new environment variables in the Docker relay section of the README.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/cli/cli.go	Adds EnvVars bindings for relay --ports and --port flags.
Dockerfile	Makes healthcheck port(s) configurable via CROC_PORTS/CROC_PORT.
README.md	Documents how to run the relay container with custom ports using the new env vars.

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