Use Docker-based GitHub Actions Workflows

[atc0005]

Replace native GitHub Actions with containers created
and managed through the atc0005/go-ci project.

This results in three workflows:

• New, primary workflow

  • with parallel linting, testing and building tasks
  • with three Go environments
    • "old stable" - currently Go 1.13.14
    • "stable" - currently Go 1.14.6
    • "unstable" - currently Go 1.15rc1
  • Makefile is not used in this workflow
  • staticcheck linting using latest stable version
    provided by the atc0005/go-ci containers

• Separate Makefile-based linting and building workflow

  • intended to help ensure that local Makefile-based
    builds that are referenced in project README files
    continue to work as advertised until a better local
    tool can be discovered/explored further
  • use golang:latest container to allow for Makefile-based
    linting tooling installation testing since the
    atc0005/go-ci project provides containers with those
    tools already pre-installed

• Add Quick Validation workflow

  • run on every push, everything else on pull request updates
  • linting via golangci-lint only
  • testing
  • no builds

Other changes:

• README

  • Add badges for recent workflows

• Makefile lintinstall recipe installs the very latest
  stable version of the golangci-lint binary instead
  of locking a specific version

  • this should reduce dependency "gardening"

fixes GH-165