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

[Jie Fu]

Hi Erik,

Thank you so much for your review and suggestions.
> Hello Jie,
>
> Looks good, just some grammatical notes on the first paragraph. Here 
> is my suggestion:
>
> Docker tests with default parameters may fail on OS versions newer 
> than oraclelinux 7.6.
> For example, they pass on Ubuntu 16.04 but fail on Ubuntu 18.04 if run 
> like this:
>
OK. I will update it next Monday since tomorrow is a public holiday in 
China.
Thanks.

Best regards,
Jie