Add Dockerfile and workflow for ARM

[scottyeager]

Adds a Dockerfile and Github workflow to build and release an rmb-peer container for the arm64 architecture. See https://github.com/threefoldtech/farmerbot/issues/11 for motivation and discussion.

Notes on the builds:

• Cross compiling is done using the base image provided by cross-rs. It's function is to provide a C toolchain targeting aarch64-unknown-linux-musl. Such a toolchain is not readily available from official sources like the Ubuntu package repos, and this appears to be a reliable source (1M+ downloads across their various builds)
• Some other options, like targeting gnu vs musl or building on a native toolchain within an emulated Alpine container, do exist. I ruled them out for the sake of consistency with the mainline builds and extremely long build times in that latter case

General notes:

• All changes are adjacent to the existing workflows, to prevent any issues or complications with the mainline builds
• The container images are tagged with a suffix -arm, for example latest-arm or v1.0.6-arm. While different architecture can coexist under a single tag, it's only possible if all are built under a single workflow run (avoided to preserve the point above)
• Any suggestions regarding how to organize the new files, in terms of directory structure or naming conventions, are welcome

[scottyeager]

Thanks @muhamadazmy. I've added the entrypoint. Looks like I'll need another review since I did that before merging :slightly_smiling_face:

[robvanmieghem]

Thanks Scott, although I very much understand

• All changes are adjacent to the existing workflows, to prevent any issues or complications with the mainline builds
• The container images are tagged with a suffix -arm, for example latest-arm or v1.0.6-arm. While different architecture can coexist under a single tag, it's only possible if all are built under a single workflow run (avoided to preserve the point above)

I personally do like multi-arch container images

[scottyeager]

Thanks for the input @robvanmieghem. I agree and am happy to merge these into unified workflows. It will add a bit more complexity, but I can solve any kinks on the forks and submit again.