push the image to GHCR on merge to main, slice 3 of M4

Tags :latest and the commit SHA, authenticated with the built-in
GITHUB_TOKEN via the packages:write permission. Making the package
public is a one-time manual step in the package settings after the
first push.

.github/workflows/image.yml

@@ -0,0 +1,33 @@
+# Builds the Dockerfile and pushes the result to GHCR on every merge to
+# main, tagged :latest (what the server pulls) and :<sha> (rollback to
+# any exact commit without a release process). The GITHUB_TOKEN is
+# enough to push packages; no extra secret needed, only the
+# packages: write permission below.
+name: Image
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  image:
+    name: Build and push to ghcr.io
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/checkout@v4
+      - uses: docker/setup-buildx-action@v3
+      - uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          tags: |
+            ghcr.io/justinzeus/spreadlab:latest
+            ghcr.io/justinzeus/spreadlab:${{ github.sha }}