[Docs Review] Define environment variables for docker images

[rsajwani]

Parent Issue #1222
Vitess github url: https://github.com/vitessio/website

Pages to review:

We should document environment variable for each docker image we produce on dockerhub. Here are all the images.

• vttestserver
• k8s
• logtail
• logrotate
• vtctld
• vtctlclient
• vtctl
• mysqlctl
• mysqlctld
• vttablet
• vtage
• vtadmin
• vtexplain
• bootstrap
• vtworker
• lite
• base
• orchestrator

Lets document all images with their respective ENV variables

• docker readme
• vttestserver
• perhaps create new page to list flags for each image just like above vtestserver image on website.

What to do

• Make sure the content reflect the current state and facts are up-to-date.
• Add details if needed either in form of new section on current page or adding new page.
• Add links/Urls on page are working
• Deprecate flags should be marked as 'deprecated'
• Review is done for both V15 and V16

Please note that we are not going to focus of restructuring the website content as part of this project. It is something we will do as a separate project later on.