Add chapel-gasnet-udp Docker image

[arifthpe]

Add a chapel-gasnet-udp Docker image variant.

Pulled from Bears-R-Us/arkouda-contrib#181.

Also includes:

• build and test of the image in util/cron/test-docker.bash alongside the other variants
• remove some boilerplate from current Docker testing
• test ./hello -nl 2 in gasnet Docker images

Note: When merged, I think we'll need to manually set up a chapel-gasnet-udp repository under the chapel Dockerhub before the first push occurs.

[reviewer info placeholder]

Testing:

• local build and test succeeds