Allow shadowbox to run as a Snap/AppImage

[sr229]

The current problem with Shadowbox is its hard dependency with outline-ss-server being in the CWD of /root, if running locally, or if by script, using Docker. Not all providers supports Docker since some of them uses LXC like SkySilk.

It would be nice if we can package it as a single executable like a AppImage or distributed easier like a Snap. Both implementations require no Docker and would allow more lesser-known providers to use Outline without the need for Docker.

[trevj]

@sr229 Thanks for the suggestion!