docker-buildx: init at 0.5.1

Installing docker-buildx enables buildx subcommand on the client:

* https://github.com/docker/buildx

pkgs/applications/virtualization/docker/buildx.nix

@@ -0,0 +1,25 @@
+{ stdenv, lib, buildGoModule, fetchFromGitHub }:
+
+buildGoModule rec {
+  pname = "docker-buildx";
+  version = "0.5.1";
+
+  src = fetchFromGitHub {
+    owner = "docker";
+    repo = "buildx";
+    rev = "v${version}";
+    sha256 = "0l03ncs1x4lhgy0kf7bd1zq00md8fi93f8xq6k0ans4400divfzk";
+  };
+
+  vendorSha256 = null;
+
+  installPhase = ''
+    install -D $GOPATH/bin/buildx $out/libexec/docker/cli-plugins/docker-buildx
+  '';
+
+  meta = with lib; {
+    description = "Docker CLI plugin for extended build capabilities with BuildKit";
+    license = licenses.asl20;
+    maintainers = [ maintainers.ivan-babrou ];
+  };
+}