cross rustembedded containers

Allows to compile (rust-)htslib in a variety of environments and architectures via rustembedded cross.

Quickstart

$ cd docker
$ docker build -t brainstorm/cross-x86_64-unknown-linux-musl:1.0.0 . -f Dockerfile.musl
$ docker build -t brainstorm/cross-x86_64-unknown-linux-gnu:1.0.0 . -f Dockerfile.gnu

Then, to build and test rust-htslib with the above containers, proceed as you would with cargo, using cross instead, i.e:

$ cross build --target x86_64-unknown-linux-musl