Add github action to build a Docker image and update for Nextflow 21.04.1

[antunderwood]

Summary of changes

• add docker-build-and-push.yml github action to build on PR and push on tags
  two containers are built
  - latest-nanopore
  - latest-illumina
  the reference to these will need updating in the nextflow.config to refer to connor-lab rather than cgps-dev
• fixed github tests action
• update NF to 21.04.1
• update syntax to use val statments e.g val(sampleName)
• WIP added nextflow_schema.json to render as a form shown in NF tower