[BUG] Improved string matching with exit-code-based detection for db connection

[JayShenkar07]

Fixes issue: #448

This PR fixes the MySQL connection retry logic in the Docker Mautic container startup script.
Currently, the startup script relies on string matching to detect whether MySQL is alive:

Problem with String-Based Matching

• Unreliable output: mysqladmin ping may print warnings, the script checks for exact string equality, any extra text (like the warning) causes the condition to never evaluate to true.
• Logical issue: Even if the database is up and reachable, the loop continues indefinitely, printing "MySQL is not ready yet" and hanging the container startup. (This is exact case what I was facing)

Changes Made

• Replaced string matching with exit-code-based detection using mysqladmin ping.
• Exit code 0 → MySQL is reachable.
• Exit code non-zero → retry.
• Added retry loop with configurable max attempts (MAX_FAILURE_COUNT) and logging.
• Avoids issues with warnings, extra messages, or output formatting changes.
• Ensures the container does not hang indefinitely if MySQL is temporarily unavailable.

Why This is Necessary

• Fixes container startup hangs when MySQL is not immediately available.
• Improves robustness and reliability of the Mautic Docker setup.
• Aligns with best practices for checking database availability in scripts.

[cibero42]

Hello,

Thank you for your contribution. Please have a look on my comments

[cibero42]

Suggested change

	log_error "MySQL is not responding after ${MAX_FAILURE_COUNT} attempts. Exiting."
	log_startup_error_header
	log_error "MySQL is not responding after ${MAX_FAILURE_COUNT} attempts. Exiting."

[cibero42]

Maintaining these error logs would be better, as they allow people to diagnose their DB.