Remove requirement for building a local openfire:latest Docker image

[viv]

At present we are required to build and tag a local openfire:latest image for this compose setup to work. It would be better if this was not the case and we could instead simply pull an Openfire image from a registry in the same way that we pull outher images in our setup (e.g. postgres).

Openfire does not yet publish an image to a registry but that will hopefully change with this:
igniterealtime/Openfire#2642

This issue is mainly to remind us to make use of any published Openfire image, and update READMEs.

[guusdk]

I think this is mostly resolved by #78