RFR: 8302888: containers/docker/TestJcmd.java fails when run as root under podman

[Severin Gehwolf]

Could I please get a review of this trivial test fix? When `podman` is being used and the test is being run as root, which some of the other container tests require, the test fails because of the not-supported `--userns=keep-id` option:


# id -u
0
# podman run --rm -ti --userns=keep-id fedora:37
Error: keep-id is only supported in rootless mode


I propose to only add it when non-root.

Testing:

- [x] Ran the test with podman as root/regular user. Both passed. It failed before when run as root.

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Commit messages:
 - 8302888: containers/docker/TestJcmd.java fails when run as root under podman

Changes: https://git.openjdk.org/jdk/pull/12673/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12673&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302888
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12673.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12673/head:pull/12673

PR: https://git.openjdk.org/jdk/pull/12673