Registry backup script.

Meant to be run on a different machine. It can be used on the same
machine and then the resulting files can be copied over.

backup/backup

 #!/usr/bin/env python3
 """Backup a container image registry."""
+
+
+import argparse
+import pathlib
+import sys
+import docker
+import requests
+
+
+def get_images(registry):
+    """Return a list of images from the registry."""
+    return requests.get(f"https://{registry}/v2/_catalog").json()[
+        "repositories"
+    ]
+
+
+def get_image_tags(registry, image):
+    """Return a list of tags for an image in the registry."""
+    return requests.get(f"https://{registry}/v2/{image}/tags/list").json()[
+        "tags"
+    ]
+
+
+def backup(docker_client, registry, image, tag, dest):
+    """Backup an image tag from the registry to a file in the destination
+    directory."""
+    full_name = f"{registry}/{image}:{tag}"
+    print(f"Backing up {full_name}.", file=sys.stderr)
+    try:
+        docker_client.images.get(full_name)
+        image_existed = True
+    except docker.errors.ImageNotFound:
+        image_existed = False
+    docker_image = docker_client.images.pull(full_name)
+    with open(dest / f"{tag}.tar", "wb") as tarball:
+        for chunk in docker_image.save():
+            tarball.write(chunk)
+    if not image_existed:
+        docker_client.images.remove(full_name)
+
+
+def backup_registry(registry, dest):
+    """Backup the images in the registry to the destination."""
+    docker_client = docker.from_env(timeout=90)
+    docker_client.ping()
+    for image in get_images(registry):
+        (dest / image).mkdir(exist_ok=True)
+        for tag in get_image_tags(registry, image):
+            backup(docker_client, registry, image, tag, dest / image)
+
+
+if __name__ == "__main__":
+    arg_parser = argparse.ArgumentParser(description=__doc__)
+    arg_parser.add_argument("registry", help="FQDN of the registry to backup.")
+    arg_parser.add_argument(
+        "destination", help="Location to store the images.", type=pathlib.Path
+    )
+    args = arg_parser.parse_args()
+    destination = args.destination.expanduser()
+    if not destination.exists() or not destination.is_dir():
+        arg_parser.error(
+            "Backup destination doesn't exists or isn't a directory."
+        )
+    try:
+        destination.touch()
+    except IsADirectoryError:
+        arg_parser.error("Can't write to the backup destination.")
+    try:
+        backup_registry(args.registry, destination)
+    except Exception as exception:
+        arg_parser.error(str(exception))
+    sys.exit()