A prebuilt image for amd64 and arm64, and the container no longer runs as root

Until now `docker compose up` built the image on the user's machine:
a compiler toolchain, every dev dependency, a few minutes of CPU — on a
Raspberry Pi considerably more. The compose files now pull
ghcr.io/tillheidrich/klarbild:1, built by .github/workflows/image.yml
for linux/amd64 and linux/arm64. `:1` follows every 1.x release, so an
update is `docker compose pull && docker compose up -d`. `build: .` is
one commented line away for anyone who prefers to build.

The Dockerfile has two stages now. The runtime stage carries the built
server, production dependencies and the migrations — no compiler, no
dev dependencies, no source tree. argon2 and sharp ship prebuilt
binaries for both architectures, so python/make/g++ are gone entirely.
It runs as the `node` user (uid 1000); a fresh named volume at /data
inherits that ownership.

Verified locally: image builds, boots against Postgres, applies all 17
migrations, prints the first-run password, passes its healthcheck,
accepts an upload as the unprivileged user and returns an A4 sheet that
measures 210.00 x 297.00 mm.

.github/workflows/image.yml

@@ -0,0 +1,62 @@
+name: Container image
+
+# Builds the image that docker-compose.yaml pulls, for amd64 and arm64
+# (a Raspberry Pi 4/5 or an ARM VPS runs the same image).
+#
+#   push to main   -> ghcr.io/tillheidrich/klarbild:edge
+#   tag v1.2.3     -> :1.2.3, :1.2, :1 and :latest
+#
+# Nothing to configure: the workflow pushes with its own GITHUB_TOKEN.
+
+on:
+  push:
+    branches: [main]
+    tags: ['v*.*.*']
+  workflow_dispatch:
+
+permissions:
+  contents: read
+  packages: write
+
+concurrency:
+  group: image-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  image:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v5
+
+      - uses: docker/setup-qemu-action@v3
+      - uses: docker/setup-buildx-action@v3
+
+      - uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ghcr.io/${{ github.repository_owner }}/klarbild
+          tags: |
+            type=edge,branch=main
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern={{major}}
+          labels: |
+            org.opencontainers.image.title=Klarbild
+            org.opencontainers.image.description=Self-hosted print studio: pictures on paper at exactly the size you asked for.
+            org.opencontainers.image.licenses=AGPL-3.0-only
+
+      - uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max