feat: source from ghcr (#176)

In an effort to improve our security posture, reference hardened image.

## Changes

- Changed FROM value in the Dockerfile to pull from hardened ghcr
images.

Co-authored-by: Grant Duncklee <grant.duncklee@cfpb.gov>
Co-authored-by: jcadam14 <41971533+jcadam14@users.noreply.github.com>

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.12-alpine
+FROM ghcr.io/cfpb/regtech/sbl/python-alpine:3.12
 
 WORKDIR /usr/app