RFR: 8294402: Add diagnostic logging to VMProps.checkDockerSupport [v2]

[Mikhailo Seledtsov]

> Add log() and logToFile() methods to VMProps for diagnostic purposes.

Mikhailo Seledtsov has updated the pull request incrementally with two additional commits since the last revision:

 - Comment about logging to stdout
 - Conditionally enable VMProps logging

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12239/files
  - new: https://git.openjdk.org/jdk/pull/12239/files/7851e5d4..881a547e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12239&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12239&range=00-01

  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12239.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12239/head:pull/12239

PR: https://git.openjdk.org/jdk/pull/12239