run-coverity-scan: use docker.py

run-coverity-scan: use docker.py

Our trusted docker wrapper allows run-coverity-scan to run with both
docker and podman.

For the "run" phase this is transparent; for the "build" phase however
scripts are replaced with a bind mount (-v).  This is not an issue
because the secret option is meant for secrets stored globally in the
system and bind mounts are a valid substitute for secrets that are known
to whoever builds the container.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>