Problem

Currently, when using the dev-tool test proxy commands, if the Docker image fails to start, the script just exits without explanation. At the moment, this can happen easily if, for example, the Docker daemon is not running, or the current user does not have permission to run docker commands.

Possible fix

Add a helpful error message that prints when the Docker command used by dev-tool to run the test proxy image returns a nonzero exit code.

Originally posted by @timovv in #19210 (comment)