Improve `sg migration squash-all` performance

Created by: efritz

Schemadoc currently takes an approach of creating a fresh Postgres database on the host, which makes generation of a fresh schema fast. We can rely on sg being in a developer environment and take the same approach to speed up the sg migration squash-all command introduced in #35831 (and possibly the regular squash command as well - both commands boot postgres in a fresh docker container).