Normal docker build gets logged as ERROR because of docker scan message

[scottkurz]

With a real basic liberty:devc build, the output looks like something bad happened (uses ERROR logging) for perfectly normal output.

[INFO] ---> 66605172afcc
[INFO] Successfully built 66605172afcc
[INFO] Successfully tagged intro-dev-mode:latest
[INFO] SECURITY WARNING: You are building a Docker image from Windows against a non-Windows Docker host. All files and directories ad
ded to build context will have '-rwxr-xr-x' permissions. It is recommended to double check and reset permissions for sensitive files
and directories.
[ERROR]
[ERROR] Use 'docker scan' to run Snyk tests against images to find vulnerabilities and learn how to fix them
[INFO] Completed building Docker image.

This is because docker build writes the docker scan suggestion to stderr.

It looks like
https://forums.docker.com/t/silence-the-snyk-message-after-docker-build/107982/3
we can do:
DOCKER_SCAN_SUGGEST=false docker build ..
to disable this behavior.

I think we should. I realize this fix would probably be in ci.common but it might be more discoverable here.