Document docker usage

[thenewguy]

It would be helpful to provide some examples for using the docker image in testing.

This is an example of the docker configuration that I am currently using to test a print client:

[docker:printer]
image = ghcr.io/istopwg/ippsample:master-ade54d0b7f74a9873e10fc9049750db9e88fcc99-1634767616
cmd = ippserver -n printer -p 631 -vvvv server
ports =
    0:631/TCP
healthcheck_cmd = ipptool -V 2.0 -tf /root/ippsample/examples/document-letter.pdf ipp://localhost:631/ipp/print /root/ippsample/examples/identify-printer.test
healthcheck_interval = 3
healthcheck_timeout = 3
healthcheck_retries = 30
healthcheck_start_period = 5

This demonstrates using the image from GHCR, the command used, how to expose ports, and a reasonable health check so testing waits for the container to be ready