fix Pod graceful termination

[keithpine]

What this PR does / why we need it

Adds exec to the entrypoint script so that the Graylog server process becomes PID 1. This allows the server to gracefaully handle Pod termination signals and shutdown quickly. Without the exec, the shell script swallows the signal (does not pass to child processes) and the Pod is forcibly terminated after the terminationGracePeriodSeconds setting (usually 2 minutes).

Compare to the upstream entrypoint script which already uses exec: https://github.com/Graylog2/graylog-docker/blob/1d774e6623d1cf34fb2c6a8fc72e22ee75787218/docker-entrypoint.sh#L91-L99

Graylog server Pod log with fix showing graceful shutdown:

{"instant":{"epochSecond":1758040439,"nanoOfSecond":194707442},"thread":"Thread-3","level":"INFO","loggerName":"org.graylog2.commands.Server","message":"SIGNAL received. Shutting down.","endOfBatch":false,"loggerFqcn":"org.apache.logging.slf4j.Log4jLogger","threadId":148,"threadPriority":5}
{"instant":{"epochSecond":1758040439,"nanoOfSecond":198652166},"thread":"Thread-3","level":"INFO","loggerName":"org.graylog2.system.shutdown.GracefulShutdown","message":"Graceful shutdown initiated.","endOfBatch":false,"loggerFqcn":"org.apache.logging.slf4j.Log4jLogger","threadId":148,"threadPriority":5}

There are no server logs without the graceful shutdown.

And k8s event logging, with fix, Pod is stopped and re-assigned in about 8 seconds:

2025-09-16T16:33:59Z   2025-09-16T16:33:59Z   1       kubelet             Normal   Killing     Stopping container graylog-server
2025-09-16T16:34:07Z   2025-09-16T16:34:07Z   1       default-scheduler   Normal   Scheduled   Successfully assigned logging/graylog-2 to <redacted>

Compared to no fix, Pod is stopped and re-assigned in 2 minutes after termination grace period:

2025-09-16T16:41:55Z   2025-09-16T16:41:55Z   1       kubelet             Normal    Killing     Stopping container graylog-server
2025-09-16T16:43:55Z   2025-09-16T16:43:55Z   1       default-scheduler   Normal    Scheduled   Successfully assigned logging/graylog-2 to <redacted>

Which issue this PR fixes

none

Special notes for your reviewer

none

Checklist

• DCO signed
• Chart Version bumped

[KongZ]

Can you solve the conflict?