Move from firejail to docker for building docs

[nobodywasishere]

Docker is what docs.rs uses, and it would give a lot more flexibility towards the environment docs are generated in, allowing for using older versions of the docs generator, adding custom libs, better security, and easier setup.

docs.rs uses rustwide, a secure execution environment built on Docker. A similar shard should be developed for Crystal for safe execution.

[nobodywasishere]

It would be beneficial as well to be able to run all the crystaldoc programs via docker with one command - if that's feasible.