Release process for container images

We need to push container images \-- notably SDK harness container images \-- to a public place with each release, so that they can be referenced and used. For example, https://hub.docker.com/r/apache/ might be an option, although Docker hub seems more geared towards containerizing container-unware content post-release.

For users who wish to build dev (or custom) container images any location is fine (personal account on docker hub, GCR, etc), but we do not want it to be a requirement for normal use of official Beam releases.

Imported from Jira [BEAM-2878](https://issues.apache.org/jira/browse/BEAM-2878). Original Jira may contain additional context.
Reported by: herohde.

Start by reviewing the release-process requirements in this issue and the linked BEAM-2878 Jira entry, then compare the proposed Docker Hub location with the project's release flow. Done means official Beam releases publish SDK harness container images somewhere public, without requiring normal users to build or host them themselves.

Written by the indexing model from the issue text.