Add mqttcool

[gfinocchiaro]

Checklist for Review

NOTE: This checklist is intended for the use of the Official Images maintainers both to track the status of your PR and to help inform you and others of where we're at. As such, please leave the "checking" of items to the repository maintainers. If there is a point below for which you would like to provide additional information or note completion, please do so by commenting on the PR. Thanks! (and thanks for staying patient with us ❤️)

• associated with or contacted upstream?
  • See
    • https://github/Lightstreamer
    • https://github/mqttcool
• does it fit into one of the common categories? ("service", "language stack", "base distribution")
  • I would say service
• is it reasonably popular, or does it solve a particular use case well?
  • See https://www.lightstreamer.com/mqttcool-features
• does a documentation PR exist? (should be reviewed and merged at roughly the same time so that we don't have an empty image page on the Hub for very long)
  • See Add mqttcool docs#1166
• dockerization review for best practices and cache gotchas/improvements (ala the official review guidelines)?
• 2+ dockerization review?
• existing official images have been considered as a base? (ie, if foobar needs Node.js, has FROM node:... instead of grabbing node via other means been considered?)
  • Using openjdk:8-jre-alpine and openjdk:8-jre
• if FROM scratch, tarballs only exist in a single commit within the associated history?
• passes current tests? any simple new tests that might be appropriate to add? (https://github.com/docker-library/official-images/tree/master/test)

[gfinocchiaro]

Added Checklist for Review

[gfinocchiaro]

@tianon @yosifkit Is there anything else I can do from my side to go on with this PR?
Thanks a lot!

[gfinocchiaro]

Thanks a lot! I've also created a new PR for docs: docker-library/docs#1275.