[FLINK-40754][docker] Add a PR template with AI disclosure - #276

SecretsUsedInArgOrEnv: Do not use ARG or ENV instructions for sensitive data (ENV "GPG_KEY") More info: https://docs.docker.com/go/dockerfile/rule/secrets-used-in-arg-or-env/

LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format More info: https://docs.docker.com/go/dockerfile/rule/legacy-key-value-format/