Support flake

[AkihiroSuda]

See examples/nginx-flake/flake.nix

Fix #1

Thanks to @tomberek for providing hints.

[tomberek]

I'm getting a permission denied during build regarding etc/group

docker-layer-nginx> Adding contents...
docker-layer-nginx> Adding /nix/store/94mnvdilyq5g6v5i6s4fj3a6bfd02rpp-fake-nss
docker-layer-nginx> Adding /nix/store/l0wlqpbsvh1pgvhcdhw7qkka3d31si7k-bash-5.1-p8
docker-layer-nginx> Adding /nix/store/fvprxgcxf4px865gdjd81fbwnxcjrg41-coreutils-9.0
docker-layer-nginx> Adding /nix/store/iscr1m6s6sakm3jqy2sc45ff6p601wvx-nginx-1.20.2
docker-layer-nginx> Adding /nix/store/l7vzwf13ls7969r5fvx36py9yx29k2bg-index.html
docker-layer-nginx> /build/.attr-0l2nkwhif96f51f4amnlf414lhl4rv9vh8iffyp431v6s28gsr90: line 16: etc/group: Permission denied

[AkihiroSuda]

@tomberek Could you open an issue with docker info and docker version?

Does docker builder prune help?

[AkihiroSuda]

Or, maybe you are trying to nix build the image without using docker build?

Then you need --option build-users-group ""

	var nixBuildCmd *exec.Cmd
	if flakeMode {
		nixBuildCmd = exec.CommandContext(ctx,
			"nix",
			"--extra-experimental-features", "nix-command",
			"--extra-experimental-features", "flakes",
			"build",
			"--option", "build-users-group", "",
			"/dockerfile",
		)
	} else {
		nixBuildCmd = exec.CommandContext(ctx, "nix-build", "--option", "build-users-group", "", fullFilename)
	}

[AkihiroSuda]

Opened PR #7 for improving nix build compatibility