Clean up various issues associated with generalising code and moving build to github.

• build: validate that version number of sample manifest files matches version.txt
• build: turn on dependabot
• build: add __version__, __gitsha__ and __build_date__ variables
• build: set :dev label on docker image for most recent build
• bugfix: retry initialisation if OaatGroup is created before its OaatType (and generally improve robustness of handlers)
• bugfix: fix login issue caused by the removal of the kubernetes package from container build
• dependencies: PyYAML 5.3 -> 5.3.1

Test release process

• CI changes only

Test release process

• should take the container already built and add version tags

Another attempt at automated docker image tagging

• CI changes only

Test automated docker image generation

• CI changes only

Add build pipeline

• On check in, GitHub Actions builds docker container and pushes container GitHub container registry
• Fixes #11

v0.4.0

Improve description of approach in README