crane: cannot push or validate .tar.gz format

[a-h]

Describe the bug

crane doesn't support working with .tar.gz files, requiring that Docker container images that are gzipped are first untarred.

Attempting to run crane validate --tarball <filename>.tar.gz results in the error:

Error: failed to read image <filename>.tar.gz: archive/tar: invalid tar header

To Reproduce

• Create an empty directory.
• Initialize a git repo in it with git init.
• Create a flake.nix file containing this content and add it with git add flake.nix:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/23.11";
  };

  outputs = { self, nixpkgs }:
    let
      allSystems = [
        "x86_64-linux" # 64-bit Intel/AMD Linux
        "aarch64-linux" # 64-bit ARM Linux
      ];
      forAllSystems = f: nixpkgs.lib.genAttrs allSystems (system: f {
        system = system;
        pkgs = import nixpkgs {
          inherit system;
        };
      });

      # Build Docker container.
      dockerImageTools = system: pkgs: pkgs.dockerTools.buildImage {
        name = "tools";
        tag = "latest";
        copyToRoot = [
          pkgs.coreutils
          pkgs.bash
          pkgs.jq
        ];
        config = {
          Cmd = [ "bash" ];
        };
      };
    in
    {
      packages = forAllSystems ({ system, pkgs }: {
        docker-image-tools = dockerImageTools system pkgs;
      });
    };
}

• Run nix build ./#packages.x86_64-linux.docker-image-tools which outputs a result file, which is a .tar.gz file.
• Run crane validate --tarball /nix/store/8cix3l8fwpz7m5v7cggmk0467s9s7naj-docker-image-tools.tar.gz to see the error.

Expected behavior

Expected it to work without additional steps.

[a-h]

This issue would be fixed by merging #1858

I've tested the PR and confirm it works well.

This also probably fixes #1838

[zhangguanzhang]

@jonjohnsonjr PTAL

[zhangguanzhang]

@imjasonh cc