RFR: 8221894: Add comments for docker tests in the test doc

[Jie Fu]

Hi all,

JBS:    https://bugs.openjdk.java.net/browse/JDK-8221894
Webrev: http://cr.openjdk.java.net/~jiefu/8221894/webrev.00/

This is a follow on from 8221342 to add comments for docker tests.

The docker tests with default parameters may fail on the os newer than 
oraclelinux 7.6.
For example, it is passed on Ubuntu 16.04 but failed on Ubuntu 18.04 with
------------------------------
make test TEST="jtreg:test/hotspot/jtreg/runtime/containers/docker"
------------------------------

To run these tests correctly, the correct docker image parameters are 
required on Ubuntu 18.04 by using "JAVA_OPTIONS".
------------------------------
make test TEST="jtreg:test/hotspot/jtreg/runtime/containers/docker" 
JTREG="JAVA_OPTIONS=-Djdk.test.docker.image.name=ubuntu 
-Djdk.test.docker.image.version=latest"
------------------------------

It would be helpful to add these comments.

Could you please review this change?
The testing.html is updated automatically using "make update-build-docs" 
with pandoc version 2.7.1.
Thanks a lot.

Best regards,
Jie