release.sh: correct the build prerequisites (builds are remote, not docker-local)

I had it wrong: `cloudron builder build` uploads the build context and the
registry app's BuildKit builds it — the caller needs no docker, only a registry
login. The guard now checks for that login and prints both bootstrap routes
(narrow app password scoped to the registry app, or the CLI device flow) instead
of blaming a missing docker binary.

release.sh

@@ -39,9 +39,28 @@ cr() { cloudron --server "$SERVER" --token "$CLOUDRON_TOKEN" "$@"; }
 manifest_tag() { python3 -c "import json;print(json.load(open('$MANIFEST'))['dockerImage'].rsplit(':',1)[-1])"; }
 
 cmd_build() {
-  command -v docker >/dev/null || die "no docker on this host — builds run where docker + the registry login live, not on the agent VM"
+  # Builds are REMOTE: the CLI uploads the build context and the registry's
+  # BuildKit does the work (no docker on the caller). What is required is a
+  # registry login for this user on this host — see the check below.
   local tag="${TAG:-$(date -u +%Y%m%d)-$(git rev-parse --short HEAD)}"
   [ -z "$(git status --porcelain)" ] || die "working tree is dirty — commit first, the tag carries the git sha"
+  if ! cloudron builder info >/dev/null 2>&1; then
+    die "no registry build config on this host (this is the only blocker to ./release.sh build)"
+  fi
+  if grep -qi "not logged in" <<<"$(cloudron builder info 2>&1 || true)"; then
+    cat >&2 <<'MSG'
+release.sh: this host has no registry login, so it cannot start a remote build.
+  Bootstrap it one of two ways:
+    a) narrow: in the Cloudron dashboard, mint an app password for registry-bot
+       scoped to the registry app (c257ba3c-6737-4375-ad51-ea19586bf3f0), put it on
+       this host, then write into ~/.cloudron.json:
+       {"builder": {"kind":"registry","type":"remote","url":"https://registry.inference.coop",
+                    "username":"registry-bot","token":"<app password>"}}
+    b) broad: `cloudron login my.inference.coop` (device flow — a human with
+       dashboard access confirms a code), then `cloudron builder login`.
+MSG
+    exit 1
+  fi
   printf 'building %s:%s (base %s)\n' "$REGISTRY_REPO" "$tag" "$BASE_IMAGE"
   cloudron builder build --no-cache --repository "$REGISTRY_REPO" --tag "$tag" || die "build failed"
   printf '\nbuilt tag: %s\nnext: ./release.sh pin %s\n' "$tag" "$tag"