Update docs concerning default image user id

[ukclivecox]

What this PR does / why we need it:

• Adds docs for clarifying the container userId

Which issue(s) this PR fixes:

Fixes #2142

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[seldondev]

Sun Sep 6 15:41:37 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2384/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2384 --build=2

[seldondev]

Sun Sep 6 15:41:41 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2384/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2384 --build=1

[adriangonz]

/approve

@cliveseldon change looks good!

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: adriangonz

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [adriangonz]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[seldondev]

Mon Sep 7 10:46:11 UTC 2020
The logs for [pr-build] [3] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2384/3.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2384 --build=3

[seldondev]

Mon Sep 7 10:46:13 UTC 2020
The logs for [lint] [4] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2384/4.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2384 --build=4