this version uses the rust-src component that rustup installs instead
of fetching the source tarball from static.r-l.o, which sometimes went
wrong due to the fallible logic that Xargo 0.1.x used
`apt update` and `apt install` should be in the same Docker RUN statement, otherwise `apt update` will be cached and `apt install` will 404 if a package no longer exists.
notable changes in the docker-based testing infrastructure
- the docker containers can now modify $CARGO_HOME, to re-use the outer
Cargo registry, and the target directory to re-use build artifacts.
- the docker containers are removed when their execution finishes
because it's no longer necessary to re-start them to inspect them
because all the interesting output is in the outer target directory
