Allow concurrent execution of hotspot docker tests

The following attribute should be removed so that the test/hotspot/jtreg/containers/docker/*.java tests can be executed concurrently. This reduces total execution from about 8 minuets to about 2.5 minutes on my machine.

https://github.com/openjdk/jdk/blob/140419fe0e8b7ff18f8a71c98e5c7372dfa3781e/test/hotspot/jtreg/containers/docker/TEST.properties#L24

exclusiveAccess.dirs=.

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In early days of JDK support for docker this was done as a precaution to avoid collisions in docker operations, such as building an image. Such collisions should no longer happen (each image is uniquely named).

In comparison, in tests/jdk/jdk/internal/platform/docker there are no such limitations, and the 6 docker tests there can be executed in parallel without issue.