doc: make sure the intended message is redirected to file successfully
doc: make sure the intended message is redirected to file successfully

- suppress the irrelated warning message
- export `stderr` besides `stdout` to the output file

Signed-off-by: Dave Chen <3fbbe9d8910afe5176d6dd2fc1de9e5729a30247@arm.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc