Build/Test Tools: Add a start period to the MySQL healthcheck on the 6.9 branch.

[adimoldovan]

Backport of r62937 to the 6.9 branch. Developed in #12753.

The mysql healthcheck sets interval, timeout, and retries, but not start_period. Docker therefore counts the checks that run while the database initializes against the retry budget of interval × retries, which is 50 seconds.

The first start of a database volume creates the data directory and runs the files in /docker-entrypoint-initdb.d. On a loaded CI runner that work can take longer than 50 seconds. Docker then marks the container unhealthy, every service that declares mysql: condition: service_healthy fails to start, and docker compose up fails:

09:33:37  Container wordpress-develop-mysql-1  Started
09:33:37  Container wordpress-develop-mysql-1  Waiting
09:34:27  Container wordpress-develop-mysql-1  Error
          dependency failed to start: container wordpress-develop-mysql-1 is unhealthy

Example run: https://github.com/WordPress/wordpress-develop/actions/runs/30439797855/job/90536164263

This patch adds start_period: 60s.

Testing instructions

The failure needs a first start that takes more than 50 seconds. A fast machine initializes the volume in about 10 seconds, so the failure does not appear locally. Measure the margin instead:

npm run env:stop
docker compose down -v
docker compose up -d mysql
docker inspect --format '{{.State.Health.Status}}' $(docker compose ps -q mysql)

Repeat the inspect command and record how long the container stays starting. Compare that time to the 50 second budget.

Then confirm that the start period does not delay a healthy start: on this branch the container still reports healthy at the same point it did before, not 60 seconds later.

Trac ticket: https://core.trac.wordpress.org/ticket/65752

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A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 62948
GitHub commit: 42ef1ae

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.