refactor: combine COPY commands

Combine two `COPY` commands to add the package.json and
package-lock.json files at the same time - the same way that is being
used for composer.json and composer.lock files earlier in the file.

tools/docker/images/Dockerfile

@@ -12,8 +12,7 @@ RUN sculpin generate --env prod
 
 FROM node:14 AS assets
 WORKDIR /app
-COPY package.json .
-COPY package-lock.json .
+COPY package*.json ./
 RUN npm ci
 COPY . .
 RUN npm run production