introduce docker

[mcluseau]

Hi strongSwan devs :)

I would like to know if there's interest in docker support upstream.

I've made some work here to build alpine-based container images. There's a Dockerfile for the "build from source case" (Dockerfile) and another one for the "build from release" (Dockerfile.release) with a build-arg to select the release to build.

They have a lot in common so they're an assembly built by docker/Makefile.

There's some preliminary work to try to use the existing tests in a docker environment, but it's not finished.

I'm opening this PR to get some feedback, as it's in WIP state (the docker is able to establish connections, though).

Thanks!

[cla-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have the users @mcluseau on file. In order for us to review and merge your code, please contact the project maintainers via info@strongswan.org to get yourself added.

[tobiasbrunner]

@cla-bot check

[cla-bot]

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