Add support for .dockerignore files

[tristanpemble]

Implements #12. This seems to work with my limited testing.

This is my first time writing or contributing anything in Go, so, caveat emptor

[AkihiroSuda]

Thanks, but looks like flake.lock is no longer copied into the dockerfile ctx?

#12 /self/buildkit-nix helper --filename=flake.nix
#12 sha256:d66b9fedc66b8d84730984cf095e9461ce0916bc23d5496de99cfb05cf950408
#12 0.276 time="2022-02-22T16:08:25Z" level=info msg="Populating cache from /cache into /nix"
#12 0.279 time="2022-02-22T16:08:25Z" level=info msg="Running [nix --extra-experimental-features nix-command --extra-experimental-features flakes build --option build-users-group  /dockerfile] (flake mode: true)"
#12 11.85 warning: creating lock file '/dockerfile/flake.lock'
#12 12.28 error: `runVend` has been replaced by `proxyVendor`
#12 12.28 (use '--show-trace' to show detailed location information)
#12 12.29 time="2022-02-22T16:08:37Z" level=fatal msg="exit status 1"
#12 ERROR: executor failed running [/self/buildkit-nix helper --filename=flake.nix]: exit code: 1

https://github.com/AkihiroSuda/buildkit-nix/runs/5291062997?check_suite_focus=true