Add Native Alpine Image

[Alexhuszagh]

Checklist

• I've looked through the issues and pull requests for similar request
• This feature could be solved with a custom docker image (optional)

Describe your request

Due to issues with C++ dynamic library linking with x86_64 hosts and an x86_64 musl target, we should probably provide an x86_64-unknown-linux-musl.alpine image. This would ensure the library is properly linked, without avoiding our main being moved off a Ubuntu base.

Describe why this would be a good inclusion for cross

This fixes #902, which is currently an issue we should probably address quickly. I'm hoping this could wait for #982, since this would allow us to target more than just x86_64 for Alpine images, but without requiring many more images to be added (only a native image for each platform).