# Pulling the nixos/nix Docker image from a specific registry

This example pulls the [`coreos/etcd` image](https://quay.io/repository/coreos/etcd) from the `quay.io` registry.

```nix
{ dockerTools }:
dockerTools.pullImage {
  imageName = "quay.io/coreos/etcd";
  imageDigest = "sha256:24a23053f29266fb2731ebea27f915bb0fb2ae1ea87d42d890fe4e44f2e27c5d";
  finalImageName = "etcd";
  finalImageTag = "v3.5.11";
  hash = "sha256-Myw+85f2/EVRyMB3axECdmQ5eh9p1q77FWYKy8YpRWU=";
}
```
:::

::: {.example #ex-dockerTools-pullImage-nixprefetchdocker}
# Finding the digest and hash values to use for `dockerTools.pullImage`

Since [`dockerTools.pullImage`](#ssec-pkgs-dockerTools-pullImage) requires two different hashes, one can run the `nix-prefetch-docker` tool to find out the values for the hashes.
The tool outputs some text for an attribute set which you can pass directly to `pullImage`.

```shell
$ nix run nixpkgs#nix-prefetch-docker -- --image-name nixos/nix --image-tag 2.19.3 --arch amd64 --os linux
(some output removed for clarity)
Writing manifest to image destination
-> ImageName: nixos/nix
-> ImageDigest: sha256:498fa2d7f2b5cb3891a4edf20f3a8f8496e70865099ba72540494cd3e2942634
-> FinalImageName: nixos/nix
-> FinalImageTag: latest
-> ImagePath: /nix/store/4mxy9mn6978zkvlc670g5703nijsqc95-docker-image-nixos-nix-latest.tar
-> ImageHash: 1q6cf2pdrasa34zz0jw7pbs6lvv52rq2aibgxccbwcagwkg2qj1q
{
  imageName = "nixos/nix";
  imageDigest = "sha256:498fa2d7f2b5cb3891a4edf20f3a8f8496e70865099ba72540494cd3e2942634";
  hash = "sha256-OEgs3uRPMb4Y629FJXAWZW9q9LqHS/A/GUqr3K5wzOA=";
  finalImageName = "nixos/nix";
  finalImageTag = "latest";
}
```

It is important to supply the `--arch` and `--os` arguments to `nix-prefetch-docker` to filter to a single image, in case there are multiple architectures and/or operating systems supported by the image name and tags specified.
By default, `nix-prefetch-docker` will set `os` to `linux` and `arch` to `amd64`.

Run `nix-prefetch-docker --help` for a list of all supported arguments:
```shell
$ nix run nixpkgs#nix-prefetch-docker -- --help
(output removed for clarity)
```
:::

## exportImage {#ssec-pkgs-dockerTools-exportImage}

This function is similar to the `docker container export` command, which means it can be used to export an image's filesystem as an uncompressed tarball archive.
The difference is that `docker container export` is applied to containers, but `dockerTools.exportImage` applies to Docker images.
The resulting archive will not contain any image metadata (such as command to run with `docker container run`), only the filesystem contents.

You can use this function to import an archive in Docker with `docker image import`.
See [](#ex-dockerTools-exportImage-importingDocker) to understand how to do that.

:::{.caution}
`exportImage` works by unpacking the given image inside a VM.
Because of this, using this function requires the `kvm` device to be available, see [`system-features`](https://nixos.org/manual/nix/stable/command-ref/conf-file.html#conf-system-features).
:::

### Inputs {#ssec-pkgs-dockerTools-exportImage-inputs}

`exportImage` expects an argument with the following attributes:

`fromImage` (Attribute Set or String)

: The repository tarball of the image whose filesystem will be exported.
  It must be a valid Docker image, such as one exported by `docker image save`, or another image built with the `dockerTools` utility functions.

  If `name` is not specified, `fromImage` must be an Attribute Set corresponding to a derivation, i.e. it can't be a path to a tarball.
  If `name` is specified, `fromImage` can be either an Attribute Set corresponding to a derivation or simply a path to a tarball.

  See [](#ex-dockerTools-exportImage-naming) and [](#ex-dockerTools-exportImage-fromImagePath) to understand the connection between `fromImage`, `name`, and the name used for the output of `exportImage`.

`fromImageName` (String or Null; _optional_)

: Used to specify the image within the repository tarball in case it contains multiple images.