Dockerize CAS server

Closes #1

See merge request !1

Setting commandLine twice in a task of type Exec was only running
the second command, never running the first one.

set to warn by default

This reverts commit aa10044d.

exec java in docker entrypoint so java is pid 1
having java be pid 1 allows for stack dump via:
  docker kill --signal=QUIT cas

put docker image in gradle.propertiees so non-slim can be used if tools
like jstack are desired