Sort keys in staticfiles.json to support reproducible builds

[Matthew Stell]

I am attempting to implement a build process whereby all my docker builds are reproducible.
I am running the collectstatic management command (using the ManifestStaticFilesStorage backend) as the final step of the Dockerfile, however the ordering of the key-value pairs in the staticfiles.json are not consistent across builds.

I propose that the keys in the staticfiles.json "paths" section are sorted alphabetically to ensure that the staticfiles.json file generated for a given set of static files is always consistent.
I do not believe that this will introduce a breaking change.

I am happy to submit a PR to implement this if accepted.

Here is the relevant code block:
​https://github.com/django/django/blob/1ba5fe19ca221663e6a1e9391dbe726bb2baaf8a/django/contrib/staticfiles/storage.py#L498

    def save_manifest(self):
        self.manifest_hash = self.file_hash(
            None, ContentFile(json.dumps(sorted(self.hashed_files.items())).encode())
        )
        payload = {
            "paths": self.hashed_files,
            "version": self.manifest_version,
            "hash": self.manifest_hash,
        }
        if self.manifest_storage.exists(self.manifest_name):
            self.manifest_storage.delete(self.manifest_name)
        contents = json.dumps(payload).encode()
        self.manifest_storage._save(self.manifest_name, ContentFile(contents))

[Sarah Boyce]

Thank you for the ticket, this is a duplicate of #35846. To progress this we need a way to reproduce so that we can later confirm the behavior is fixed