Create a CircleCI OpenJDK convenience image based on Eclipse OpenJ9

Eclipse OpenJ9 is another OpenJDK implementation of the JVM. It has some nice features, such as AOT, fast start-up and low memory footprint (it uses 66% less memory than Hotspot).
Since we have an OpenJDK based on Hotspot, I thought it’b be nice to also have this choice on the base CircleCI images.

See also:

Hi Gabriel. While we are planning changes that will make this easier, we have no plans to add additional convenience images.

I recommend creating your own image, following the steps here.