Add "build from github.com/postgres/postgres" variation

[vlsi]

Hi,

It is great to have dockerfiles for the released versions. However, I wonder if you could consider adding "build from master" variation.

It does not need to be rebuild on every commit, however, it would simplify testing unreleased PostgreSQL versions and features.
WDYT?

[tianon]

A parallel/similar request to this one is docker-library/golang#464 (which has some useful discussion on it including some constraints this would have to follow such as not auto-updating "too often")

I'd also like to avoid creating our own name for this thing, especially if there's already a common term in use in the PostgreSQL community for describing this (for example, in Go, it's common to refer to "Go tip" as the latest development version, to the point of having a whole website deployed on https://tip.golang.org/ for the in-development version).

[tianon]

To my second point, it appears "devel" is probably the right term: https://www.postgresql.org/docs/devel/index.html