WIP: add a Dockerfile to build docker images of Sequoia

This will make installing Sequoia easier for new users who are not familiar with Cargo or Rust toolchains. It can provide an easy, binary build for the sq command as well.

Unfortunately, the build suffers from a bug in Cargo (or Debian?) that segfaults when downloading stuff. But it generally ends up working eventually. I'm also unable to run the sq command which fails with:

thread 'main' panicked at 'internal error: entered unreachable code', tool/src/sq.rs:461:14

It's unclear to me if this is a bug in sequoia or the build process peculiar to Docker.