fix: respect Docker healthcheck start period

[stanleycyang]

Summary

• Stops delaying the first Coolify healthcheck poll until after Docker's start period
• Treats Docker's starting status as start-period time that does not consume Coolify retry attempts
• Adds a regression test for the retry-counting behavior

Fixes #2332

Test Plan

• Not run locally: this environment does not have PHP/composer installed (php, composer, and docker are unavailable).