Split image publishing into stable and nightly channels

.github/workflows/docker-publish.yml

@@ -4,6 +4,10 @@ on:
   push:
     branches: [master]
     tags: ['v*.*.*']
+  # Nightly rebuild of master: picks up base-image (security) updates even
+  # when no code changed. Publishes the same nightly tag.
+  schedule:
+    - cron: '0 4 * * *'
 
 env:
   REGISTRY: ghcr.io
@@ -41,13 +45,21 @@ jobs:
         uses: docker/metadata-action@v5
         with:
           images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          # Tagging scheme:
+          #   release tag vX.Y.Z -> X.Y.Z, X.Y, X and latest (stable channel)
+          #   master push        -> nightly, master and the commit sha
+          # `latest` therefore always points at the last tagged release,
+          # never at an arbitrary master build (flavor latest=auto only
+          # applies it on semver tag events).
+          flavor: |
+            latest=auto
           tags: |
-            type=ref,event=branch
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
             type=semver,pattern={{major}}
+            type=raw,value=nightly,enable={{is_default_branch}}
+            type=ref,event=branch
             type=sha,prefix=
-            type=raw,value=latest,enable={{is_default_branch}}
 
       - name: Build and push Docker image
         uses: docker/build-push-action@v6

README.md

@@ -37,6 +37,18 @@ docker run -d \
 # Access the web interface at http://localhost:8080
 ```
 
+#### Image tags
+
+| Tag | Meaning |
+|-----|---------|
+| `latest` | Last tagged release (stable channel) |
+| `X.Y.Z`, `X.Y`, `X` | Specific release versions |
+| `nightly` | Latest master build, also rebuilt daily for base-image updates |
+| `<sha>` | Exact commit builds |
+
+Releases are cut by pushing a git tag: `git tag v2.0.0 && git push origin v2.0.0`.
+Images are only published after the full test suite passes.
+
 Or with Docker Compose:
 
 ```bash