Add Dockerfile

[nagius]

This PR add the files needed to run MLDonkey with Docker using Ubuntu base image.

The Dockerfile is a multi-stage build that will compile the latest version of MLDonkey for the user's architecture, but will not bring development tools in the runtime image. And example of usage is provided with the docker-compose.yml file.

Note: This PR require https://github.com/ygrek/mldonkey/pull/64 to be merged to run with docker-compose properly.

[DioxCorp]

I have been a user of MLDonkey for many years, for me it is the best ED2k network client for linux and especially for microserver. I have the problem now in the most recent Ubuntu or Debian distros, which is no longer in the repositories and it is foreseeable that it will not be there again. An AppImage or similar version would be possible, since Docker is totally unknown to me and not I find hardly any info on it.

[lulol]

@nagius Dockerfile fails to compile with the default ubuntu (ubuntu:latest) as is currently setup.

ubuntu:latest uses OCaml version 4.13.1 configured with -force-safe-string while the mldonkey master use unsafe-string in configure.in options. #46

To avoid the trouble to anyone attempting to buid the container and until the master is fixed may be a good idea to pin the builder version to ubuntu:focal for which the build scripts download OCaml 4.01 directly and works fine again.

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@DioxCorp you better open a separate issue with a feature request, but I wouldn't hold my breath. It's unfortunate that the MLDonkey package was dropped, but note that you won't need to use a Docker container to build this once the compilation issue with the new OCaml versions is solved. And then just need to clone the mldonkey repo, enable the gui option and compile it yourself.