Reorganize the images under the softwareheritage hub repo

• merge Dockerfiles into a single one using multistage build
• create a replayer-specific image
• add name: to configs so it's a bit easier to deploy a config update (well it's sure a bit easier to document this actually),
• add missing wait-for-it statements in replayer/entrypoint.sh

The reason for the docker image renaming is to make it easier to
organise software heritage images on the docker hub; we now have only
one "entrypoint".