Bundle licenses into container images

Licenses like 3-clause BSD or Unicode v3 include a requirement that the license be included with any binary distributions of software (i.e., Janus) that uses dependencies covered by those licenses (i.e. our numerous deps that use these).

My reading of these licenses is that we do not need to worry about redistributing them with Janus sources: we don't distribute the source code of dependencies. Some entity using Janus does when they run cargo build/test/run. This applies to the crates we ship: we distribute only source code to crates.io and docs.rs, so that's fine.

However the binary artifacts we distribute should include Janus' license, as well as the licenses of dependencies.

For container images, David Cook suggested putting them in /usr/share/common-licenses or /usr/share/*/copyright. We could invent some build step that walks the dep tree, extracts licenses and blats them into container images... or a cheekier fix would be to include every license enumerated in licenses.allow in Janus' deny.toml.

Start by reviewing deny.toml, especially licenses.allow, and the container-image build definitions. Check which binary images are distributed and how Janus and dependency licenses are currently collected. Done means the distributed container images include Janus' license and the required dependency licenses in a documented location such as /usr/share/common-licenses.