Switch to DockerHub for Elasticsearch image

Stan Hurequested to merge
sh-use-dockerhub-elasticsearch into master

From https://gitlab.com/gitlab-org/gitlab/-/jobs/378855332, we were being rate-limited by docker.elastic.co:

Unavailable: Back-end server is at capacity (executor_docker.go:188:0s)
elasticsearch : 5.6.12
Edited by Stan Hu

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