Save SHA to public folder

This allows us to GET `/sha` to see which version of the code is
deployed to an env.

Author: misaka

.github/workflows/build-and-push-image.yml

@@ -65,6 +65,8 @@ jobs:
         uses: actions/checkout@v4
         with:
           ref: ${{ inputs.git-sha || github.sha }}
+      - name: Write build SHA
+        run: git rev-parse HEAD > public/sha
       - name: Build Docker image
         run: docker build -t "mavis:latest" .
       - name: Save Docker image