[Gpg4win-devel] docker build for CI

[Doug Richardson]

# Summary
I made a Docker based build to make it easier to build Gpg4win from scratch on a clean Debian instance. I'd like to know if there is interest in taking any of these commits upstream.

Details below.

# docker-build branch

    https://github.com/drichardson/gpg4win/commits/docker-build

To use:
1. Install Docker
2. Checkout the branch
3. Run: ./build-aux/docker-build/build.sh

The docker-build branch consists of 2 commits:
1. A commit to add the Docker build.
2. A commit to add the use of a package cache to the Docker build. While this is not required for (1), it does makes the build more reliable. When I was testing, ftp.simplesystems.org (the libpng host) was down, which caused download.sh to fail. On a developer's machine, downloaded packages are cached so this isn't much of an issue, but when building from scratch every host is hit every build so the likelihood of a download.sh failure increases.

The package cache is here:

    https://github.com/drichardson/gpg4win-packages

# github-build-action branch

    https://github.com/drichardson/gpg4win/commits/github-build-action

github-build-action demonstrates how docker-build can be used to integrate with a CI system such as GitHub Actions.

You can view the result of the CI runs here:

    https://github.com/drichardson/gpg4win/actions/workflows/build.yml

If you drill down to a specific build, you can download the gpg4win installer (at least until GitHub's artifact retention period is reached).

Doug