feat(ci): publish multi-arch Docker image to GHCR (#88)

Issue #88's biggest blocker was that docs and README pointed at
ghcr.io/mbombeck/healthlog:latest, but no workflow ever pushed an
image — every self-hoster trying to follow the docs hit "manifest
unknown".

This adds .github/workflows/docker-publish.yml. The workflow:

- Triggers on push to main, on v* tags, on PRs (build only, no push),
  and via workflow_dispatch
- Builds linux/amd64 + linux/arm64 multi-arch images via buildx
- Tags with `latest` (main only), semver (`1.2.0`, `1.2`), branch name,
  PR number, and short SHA — using docker/metadata-action defaults
- Attaches SLSA build provenance and an SBOM to every image
- Caches build layers in GitHub Actions cache for fast iterations
- Uses concurrency group + cancel-in-progress so superseded runs stop

The bundled docker-compose.yml now references
`ghcr.io/mbombeck/healthlog:latest` as the primary `image:` and keeps
`build:` as a fallback so contributors can still
`docker compose up --build` against their working tree.

SECURITY.md gains a Supply Chain section explaining where images come
from, how to verify provenance and SBOMs, and how to pin a specific
version. README's Quick Start updated to reflect that no build is
needed for self-hosters.

The first run of this workflow on main will publish the image; until
then `docker compose up` will fall back to the `build:` block.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/docker-publish.yml

@@ -0,0 +1,78 @@
+name: Build & Publish Docker Image
+
+on:
+  push:
+    branches: [main]
+    tags: ["v*"]
+  pull_request:
+    branches: [main]
+  workflow_dispatch:
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+# Cancel in-progress runs when a new commit is pushed to the same ref.
+concurrency:
+  group: docker-publish-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build-and-push:
+    name: Build & push multi-arch image
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up QEMU (multi-arch)
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      # Skip login on PRs from forks — they don't have package:write.
+      # Image is built but not pushed in that case.
+      - name: Log in to GitHub Container Registry
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Compute image tags & labels
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          tags: |
+            type=ref,event=branch
+            type=ref,event=pr
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=sha,prefix=sha-,format=short
+          labels: |
+            org.opencontainers.image.title=HealthLog
+            org.opencontainers.image.description=Self-hosted health-tracking PWA
+            org.opencontainers.image.licenses=AGPL-3.0-only
+            org.opencontainers.image.source=https://github.com/${{ github.repository }}
+
+      - name: Build & push image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          file: ./Dockerfile
+          platforms: linux/amd64,linux/arm64
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          provenance: true
+          sbom: true