Add a simple way to run grpc-java test inside emulated ARM64 docker container

[jtattermusch]

Not ready to merge, but demonstrates a neat way how to test grpc-java on x86 using a docker container and ARM emulator with just a few commands.

FTR I tried this on my linux workstation and the entire build & tests took ~40mins (which is reasonable considering the fact that the ARM environment is being emulated).

CC @ejona86

[ejona86]

Why do you say this isn't ready for merge?

It's pretty reasonable already, although I do think I'd suggest changing it to be closer to run_in_docker.sh: it shouldn't hard-code what to run and instead you should pass the command you want to run. The chmod stuff isn't necessary here though, as it is its own environment that doesn't need to mix with the normal development. But we probably do want to keep the workdir in a bind-mounted folder so that the build results can be dug through if there are failures or the like. It'd be harder, but we could also configure gradle to write to a different output directory (using -PbuildDir=build-arm or putting it in ~/.gradle/; it looks like that exposes some build script issues though).

But I'd be okay if those sorts of things were done in follow-ups as they prove helpful.

[jtattermusch]

Ok, this is now ready for final review.

• grpc-java workspace gets used directly (no copies)
• it now sets the docker --user option to avoid polluting the grpc-java workspace with files owned by root
• overall I'm happy that the solution ended up being quite minimalist and it's still pretty easy to use.