CI: use built image for conformance tests

[skriss]

Description:

Right now, our CI builds a Docker image twice -- once as part of the make conformance task, and once as part of the Docker build/push steps. We should de-dupe this so we build the image once, use it to run conformance, and then push that same image to Docker Hub if it's a push build.

[skriss]

Actually looks like we don't build the image a second time unless it's a push build to main, but we do still build the binaries for multiple arches twice per the above description.