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.

docker-compose.yaml

@@ -1,6 +1,6 @@
 # Klarbild — the default stack: the print studio and a database. That is all.
 #
-#   cp .env.example .env     # fill in the three secrets at the top
+#   cp .env.example .env     # fill in the values at the top; the file says which
 #   docker compose up -d
 #   docker compose logs app  # the first-start password is printed once, here
 #
@@ -9,9 +9,11 @@
 
 services:
   app:
-    build:
-      context: .
-      dockerfile: Dockerfile
+    # The prebuilt image (amd64 and arm64). `:1` follows every 1.x release, so
+    # `docker compose pull && docker compose up -d` is the whole update.
+    image: ghcr.io/tillheidrich/klarbild:${KLARBILD_VERSION:-1}
+    # Rather build it yourself from this checkout? Swap the line above for:
+    # build: .
     environment:
       PORT: "4321"
       DATABASE_URL: "postgres://klarbild:${DB_PASSWORD:?set DB_PASSWORD in .env}@db:5432/klarbild"