Actually remove the `images build --pull option`

This option is mostly useless to us in CI, as it attempts (and fails) to
update the base image when building the dev image. For good reason: the
base image of the dev image is the latest openedx, which has not been
pushed to docker.io at the time of building -_-

CHANGELOG.md

@@ -5,7 +5,7 @@ Note: Breaking changes between versions are indicated by "💥".
 ## Unreleased
 
 - [Bugfix] Fix "Invalid Request" error during SAML authentication (thanks @Ant1x!).
-- [Feature] Add `images build --pull` option to update base images.
+- [Feature] Add `make pull-base-images` command 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.

Makefile

@@ -70,6 +70,10 @@ release-push:
 
 ###### Continuous integration tasks
 
+pull-base-images: # Manually pull base images
+	docker image pull docker.io/ubuntu:20.04
+	docker image pull docker.io/python:3.7-alpine
+
 ci-info: ## Print info about environment
 	python --version
 	pip --version

tutor/commands/images.py

@@ -52,12 +52,6 @@ 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-    help="Always attempt to pull a newer version of the base images",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,
@@ -66,7 +60,6 @@ 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 = []
@@ -78,8 +71,6 @@ 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)