fix(ci): build without BuildKit — runner has no buildx

The CI host builds with the legacy Docker builder (buildx/BuildKit missing on
the runner). Drop the '# syntax' directive and the 'RUN --mount=type=cache'
npm cache mount from the Dockerfile, and remove DOCKER_BUILDKIT=1 from the
workflow. Restores a green build so registry.alwisp.com/jason/mrp-qrcode:latest
publishes and PORT can roll the container off the pinned old digest.

.gitea/workflows/docker-build.yml

@@ -8,6 +8,8 @@ on:
 jobs:
   build:
     # Runs on the forgerunner host: bundled Docker CLI + mounted /var/run/docker.sock.
+    # The host builds with the legacy Docker builder (buildx/BuildKit is not installed
+    # on the runner), so the Dockerfile avoids BuildKit-only features.
     runs-on: host
     steps:
       - name: Checkout
@@ -25,13 +27,6 @@ jobs:
           password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Build and Push
-        env:
-          # This Dockerfile uses BuildKit features (`# syntax=docker/dockerfile:1.7`
-          # and `RUN --mount=type=cache`). Without this the CLI falls back to the
-          # legacy builder, which ignores the syntax directive and fails at the first
-          # `--mount`. DOCKER_BUILDKIT=1 activates dockerd's integrated BuildKit
-          # backend (no separate buildx plugin required).
-          DOCKER_BUILDKIT: "1"
         run: |
           IMAGE="registry.alwisp.com/${{ gitea.repository }}"
           GIT_SHA="$(git rev-parse --short HEAD)"

Dockerfile

@@ -1,12 +1,12 @@
-# syntax=docker/dockerfile:1.7
-
 # --- deps ------------------------------------------------------------------
 FROM node:20-alpine AS deps
 WORKDIR /app
 RUN apk add --no-cache openssl libc6-compat
 COPY package.json package-lock.json* ./
-RUN --mount=type=cache,target=/root/.npm \
-    if [ -f package-lock.json ]; then npm ci; else npm install; fi
+# Plain RUN (no BuildKit cache mount): the CI host builds with the legacy Docker
+# builder — buildx/BuildKit is not available on the runner — so `--mount=type=cache`
+# is not supported here.
+RUN if [ -f package-lock.json ]; then npm ci; else npm install; fi
 
 # --- build -----------------------------------------------------------------
 FROM node:20-alpine AS builder