New release

To make and publish new docker images on docker hub, maintainers should create branch under apisix-docker repo with specific name.

For apisix and dashboard new version, the branch name should use prefix release/apisix and release/dashboard separately (e.g. release/apisix-2.6, release/dashboard-2.6). Remember to delete the release branch after the new images published, since the branch would got no use afterwards.