[hf] pin signald & signalboost docker images to `latest` tag

context:
* i (aguestuser) had mistakenly assumed that omiting the tag defaulted
  to `latest`. in fact, it means "if you have an image of any version
  for this container, that's fine, just use it!"
* that can result in containers that lag significantly and is
  confusing to debug

changes:
* so... let's pin all containers in all docker-compose files to `latest`!

note:
* at some point, we might find it preferable to pin to the actual
  sever tags we use to version our images in the container registry,
  but for now it seems more likely to introduce errors by having to
  keep track of that, so let's not until we have evidence we might
  want to. :)