build: documentation for the new docker image release process

At the time of commit this file contains some things that are not yet true
due to the dockerhub configuration not having been updated. This therefore
remains aspirational until merged, after which the docherhub configuration
will be updated.

scripts/docker-release/README.md

@@ -15,10 +15,23 @@ a Terraform release already on releases.hashicorp.com, the build will fail.
 
 ## What it produces
 
-This configuration is used to produce the "latest", "light" and "beta" tags
-in Dockerhub, as well as specific version tags.
+This configuration is used to produce the "latest", "light", and "beta"
+tags in Dockerhub, as well as specific version tags.
 
-"latest" and "light" are synonyms, and are built from a branch in this
-repository called "stable". "beta" is built from a branch called "beta". Both
-of these branches should be updated only to _tagged_ commits, and only when
+* "latest" and "light" are synonyms, and are built from a branch in this
+repository called "stable".
+* "beta" is built from a branch called "beta".
+
+All of these branches should be updated only to _tagged_ commits, and only when
 it is desirable to create a new release image.
+
+## The `full` and `master` images image
+
+This configuration does not produce the "full" image. That is instead produced
+by the `Dockerfile` in the repository root, driven by updates to the "stable"
+branch.
+
+The "master" tag is updated for _every_ commit to the master branch of
+the Terraform core repository. It is not recommended to use these images for
+any production use, but they can be useful for testing bleeding-edge features
+that are not yet included in a release.