Add container image build workflow

[marcransome]

Overview

The addition of a prebuilt development-focused container image would simplify development activities outside of the GitHub ecosystem.

Intended outcome

To provide a development-focused container image alongside the existing Codespaces prebuild, primarily for use with container engines such as Docker or Podman, outside of the GitHub ecosystem.

Tasks

• Add container image build workflow #309
• Update 'Contributing' section of README.md