RFR: 8294740: Add cgroups keyword to TestDockerBasic.java

[Ioi Lam]

Please review this trivial fix.  TestDockerBasic.java uses cgroups features so it should be tagged with `@key cgroups`.

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Commit messages:
 - 8294740: Add cgroups keyword to TestDockerBasic.java

Changes: https://git.openjdk.org/jdk/pull/10547/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10547&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294740
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10547.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10547/head:pull/10547

PR: https://git.openjdk.org/jdk/pull/10547