Improve Docker image for testing/evaluation

[benbjohnson]

As mentioned on this Fly.io Community Board post, the LiteFS Docker image doesn't run on its own since it doesn't include the fuse3. The Docker image has always been used as a packaging tool since users need to copy the litefs binary out of it. However, it does seem useful to allow users to test it in isolation without setting up their own Docker image.

This will require switching from a scratch image to an alpine image so we can import fuse3 and any other packages needed.