Docker-based cross-compilation

[mbmccoy]

This PR adds an example showing cross compilation to a raspberry pi using a Docker environment.

[tgolsson]

Hmm, what is the issue you're tackling?

[mbmccoy]

The ultimate goal I'm looking to solve is cross-compilation for the RPi (using docker). I was debugging some issues and noticed that a directory called {cache_path} was created in the sandbox, hence the original title of the draft PR.

So the good news is that the docker builds seem to work, in the sense that I can run the compiler in the docker environment! The bad news is that I don't know how to get the built artifacts into the dist directory. (Usually when I build a PEX binary it will land in dist/path/to/binary.pex.)

[tgolsson]

19:45:34.86 [INFO] Completed: Pulling Docker image cross-compiler because the image is missing locally.

Something weird with the setup? Or does this require manually building the image locally first?

[mbmccoy]

Ok, this may take me a little bit to figure out. We use the docker environment specified in a docker image build rule, and I thought it would automatically build the environment—that doesn't seem to be the case.

I can definitely locally build the docker file and then it runs, but I'll need to read up on docker environments to really understand what's going on.

[mbmccoy]

Note: This is for testing to see if the issues are fixed in the current stable; will move to a different PR before this is merged.