* Add Dockerfile for building the firmware
Setting up a local environment for building the firmware can be a
painful process. This wraps that process up in a Dockerfile containing
all the deps needed which is then used in the justfile to build the
firmware.
* Add just targets for signing and cleaning
* Change sha target to take a sha and verify it directly
* Add docs for verifying the firmware SHA sum
* Add version param to sign just target
* Update verify-sha output to be more explicit
Alex Sears