Suite release publishes a new stable Conjur image

[izgeri]

With each new suite release, we're indicating to our end users the Conjur version we suggest they use in their own deployments.

To make this as easy as possible, we should add a stable tag to the Conjur DockerHub image that is published on a new suite release.

Given the suite versioning scheme, for a suite release version v1.x.y+suite.1, when the suite release is published in the final step of the release process it would trigger publishing the Conjur version as a stable image:

docker pull cyberark/conjur:1.x.y
docker tag cyberark/conjur:1.x.y cyberark/conjur:stable
docker push cyberark/conjur:stable

Details of how publishing the suite release could trigger a stable image to be published are TBD, but would require a Jenkins pipeline due to the required DockerHub privileges.