build: use npm ci for clean, reproducible installs (#839)

Switch pr-build and the Docker image from `npm install` to `npm ci` (`npm ci --omit=dev` in the image), and enable setup-node's npm cache. Deterministic installs from the committed lockfile; CI now fails fast on an out-of-sync lockfile.

Depends on the lockfile being tracked (PR #838) — npm ci requires a committed package-lock.json.

docker/Dockerfile

@@ -10,8 +10,8 @@ RUN npm install --global pm2
 # Utilise Docker cache to save re-installing dependencies if unchanged
 COPY ./package*.json ./
 
-# Install dependencies
-RUN npm install --production
+# Install dependencies (npm ci = clean, reproducible install from the lockfile)
+RUN npm ci --omit=dev
 
 # Copy all files
 COPY ./ ./