…mages (#444)

[![Checks](https://github.com/fastly/pebble/actions/workflows/checks.yml/badge.svg)](https://github.com/fastly/pebble/actions/workflows/checks.yml)

[![Tests](https://github.com/fastly/pebble/actions/workflows/tests.yml/badge.svg)](https://github.com/fastly/pebble/actions/workflows/tests.yml)

As a follow-on to #439 this
change request implements the first step of building a new CI system for
Pebble. As this is a testbed environment for Boulder, some of these
changes may prove to be useful in the latter context.

Per
#434 (comment),
GitHub Actions is a reasonable choice for a new CI implementation to
supersede Travis. Much, but not all, of the existing functionality is
present in this initial set of changes.

Items currently implemented include:
- Binary production of `pebble` and `challtestsrv` for platforms:
  - AMD64
    - darwin
    - linux
    - windows
  - ARM64
    - darwin
    - linux
    - windows
- Multiplatform docker images published on the GitHub Container Registry
for platforms:
  - linux/amd64
  - linux/arm64
  - windows/amd64
- Golang linting.
- Golang coverage.
- Local test scripts for Go and Docker:
  - `./build.sh`
  - `./docker.sh`
  - `./test.sh`

This PR also adds a `-version` flag to Pebble, which is set during
release build to the Git commit ref.

This change builds on earlier work done in
#379 and
#442.