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.

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