Dockerfile: Add sidecar support

[saleemrashid]

This PR adds support for using the LiteFS container as a sidecar, when used in a container runtime that supports mount propagation (e.g. filesystems mounted inside one container can be propagated to the host or other containers).

• It adds the required fusermount3 binary
• It adds a -skip-unmount argument to litefs mount, and uses that as the container command. When mounting on top of an existing mountpoint, LiteFS would unconditionally unmount it. This would break mount propagation, so we want to avoid this behavior.

Fixes #365