Add images build --pull option to pull base images on build

This will be useful in CI, where we want to automatically update base
images as soon as a security fix comes out.

CHANGELOG.md

@@ -4,6 +4,7 @@ Note: Breaking changes between versions are indicated by "💥".
 
 ## Unreleased
 
+- [Feature] Add `images build --pull` option to update base images.
 - [Improvement] Annotate types all over the Tutor code base.
 - [Bugfix] Fix parsing of YAML CLI arguments that include equal "=" signs.
 - [Bugfix] Fix minor edge case in `long_to_base64` utility function.

tutor/commands/images.py

@@ -52,6 +52,12 @@ def images_command() -> None:
     "--target",
     help="Set the target build stage to build.",
 )
+@click.option(
+    "--pull",
+    "pull_base_images",
+    is_flag=True,
+    help="Always attempt to pull a newer version of the base images",
+)
 @click.pass_obj
 def build(
     context: Context,
@@ -60,6 +66,7 @@ def build(
     build_args: List[str],
     add_hosts: List[str],
     target: str,
+    pull_base_images: bool,
 ) -> None:
     config = tutor_config.load(context.root)
     command_args = []
@@ -71,6 +78,8 @@ def build(
         command_args += ["--add-host", add_host]
     if target:
         command_args += ["--target", target]
+    if pull_base_images:
+        command_args.append("--pull")
     for image in image_names:
         build_image(context.root, config, image, *command_args)