Support for uid mapping

Remove dependency on fusermount3, now dockerc works on any system with a linux kernel 4.18 or above (all supported kernels are above that version).

aarch64/arm64 support!!! Specify the target architecture with --arch.

dockerc binaries for arm64 are not provided yet.

Fix https://news.ycombinator.com/item?id=39621584

Full Changelog: v0.2...v0.2.1

Full Changelog: v0.1...v0.2

Removed bug that prevents unmounting when runtime exists