The container ecosystem is growing. It would be good to be able to use custom command to figure out whether a specific host type allows for container testing; hence this enhancement seeks to make the command used by jtreg "requires" extension configurable.

--------- Details:
File containing implementation: test/jtreg-ext/requires/VMProps.java
Methods: dockerSupport(), checkDockerSupport()
(perhaps a good idea to rename these methods to containerSupport)

Proposing to create a property that would specify a command; current behavior will be default if property is not specified or empty.
For instance: jdk.test.container.requires.check.command