Use reproducible digest SHA for Integration testing docker images

[devinrsmith]

Right now, Integration testing inherits a couple of docker images that aren't pinned. It appears to be postgres:latest and docker.redpanda.com/redpandadata/redpanda:v23.2.8, but further investigation should be done to make sure this is the full scope. These should instead use our docker registry tooling to use a specific digest SHA

Noticed during creation of #5901