Use InfinispanServerRule externally

We are trying to use InfinispanServerRule on jdg-functional-tests but we can only start the tests with container mode if we set

-Dserver.output.dir=jdg/server/runtime/target/rhdg-server-8.0.0

server.output.dir needs to use a relative path inside jdg project, otherwise docker container doesn't start. This is happening because this line https://github.com/infinispan/jdg/blob/13cae7b461f934f7349b776b6269f95ce3ecbcdd/server/runtime/src/test/java/org/infinispan/server/test/ContainerInfinispanServerDriver.java#L123
In other words, If we download redhat-datagrid-8.0.0-server and set -Dserver.output.dir to server downloaded docker container doesn't start