docker.cmd.test doesn't mention stderr

[ashb]

The text for docker.cmd.text says:

How to run the bundled test-suite for the image under the Docker runtime. MUST execute tests then exit, returning output on stdout and exit with a non-zero exit code on failure

It it conceivable that a test suite might only print errors on stderr, and nothing on stdout.

Also is the intent that all these are MUSTs?

• execute tests then exit
• returning output on stdout
• exit with a non-zero exit code on failure

(I think so, but the wording could perhaps be clarified.)