COE Container builds automated on DockerHub

COE-30 laid the technical ground work to make CNI container image available on the Quay.io container registry, and that is now done and "works" - but here is what is left to "productise" it for real use:

1. https://quay.io/repository/opendaylight/ organization access should be granted to the other CoE committers and the Linux Foundation? I'm happy to add anyone who would like to be added - just email me your ID on Quay.io ...

2. Some... Jenkins Job needs to trigger the webhook on Quay ... see comment in COE-30. An easier alternative do this could be if instead of pointing it to https://git.opendaylight.org/gerrit/coe.git we pointed it to https://github.com/opendaylight/coe and https://github.com/orgs/opendaylight/people would accept the built-in one-click Web Hook that it can set up which just works out of the box. Do note https://docs.quay.io/guides/github-read-only.html

3. I'm not yet crystal clear how we'll publish different versions. Probably just tags matching ODL major release branches?