Licensing the `docker-rust` repository

[Kobzol]

Me and @Muscraft would like to add a license to this repository. Multiple people have asked for this (https://github.com/rust-lang/docker-rust/issues/12, https://github.com/rust-lang/docker-rust-nightly/issues/14), and it is a good idea overall. The effort should be quite similar to the recent licensing of rustc-perf.

I think that we can use our default Rust Project license, i.e. a combined MIT/Apache-2.0 license. As usually, we will need to contact contributors that have contributed code to this repository previously, to get their permission. Furthermore, since this repository contains code from https://github.com/rust-lang/docker-rust-nightly (which has been since archived), we should also ask people who contributed to that repository for their approval. I ran my script on these two repositories, it should be 21 people in total.

I would propose the following approach:

1. Create a PR that adds a license, so that people can check the license wording (there is already https://github.com/rust-lang/docker-rust/pull/74, but I'm not sure if @TimmFitschen is still around, if not, we can create a new one).
2. Create a new issue, like this one, where we will ask contributors for their approval, with the following text:

I license my past contributions to the `rust-lang/docker-rust` and `rust-lang/docker-rust-nightly` repositories under the dual MIT/Apache-2.0 license, allowing licensees to chose either at their option.

3. Wait until we get the approval of contributors, and then merge the PR with the new license.

@abibroom Any thoughts/concerns?

[abibroom]

Sounds good to me!

[TimmFitschen]

I updated #74 . Please use it if you like.

[Kobzol]

The repository has been licensed (https://github.com/rust-lang/docker-rust/issues/207).