dev/sg: implement 'sg ops update-images -k compose'

Created by: bobheadxi

Allows sg ops update-images to work on Compose manifests.

From a hack session with @davejrt where we tried to deploy otel-collector in a compose deployment - to try opentelemetry we needed the latest insiders images.

Test plan

go build -o ../deploy-sourcegraph-docker/sg ./dev/sg

then in deploy-sourcegraph-docker:

./sg -v ops update-images -k compose -t insiders ./docker-compose