add universal module

[AtomicFS]

It should be quite easy to add a universal module, kinda like edk2 or u-root is at the moment - it just runs user-defined command is a docker container.

This way people could use firmware-action even for yet-unsupported projects, or even some non-firmware projects and steps.

Instead of:

"build_command": "cmd"

it would use:

"build_commands": [
  "cmd1",
  "cmd2",
]