convert yarn commands to npm commands in dockerfile

Dockerfile

@@ -11,10 +11,10 @@ WORKDIR /app
 
 # only copy package.json initially so that `RUN yarn` layer is recreated only
 # if there are changes in package.json
-ADD package.json yarn.lock /app/
+ADD package.json package-lock.json /app/
 
-# --pure-lockfile: Don’t generate a yarn.lock lockfile
-RUN yarn --pure-lockfile
+# --no-save: Don’t generate a package-lock.json lockfile
+RUN npm install --no-save
 
 # copy all file from current dir to /app in container
 COPY . /app/
@@ -23,4 +23,4 @@ COPY . /app/
 EXPOSE 4040
 
 # cmd to start service
-CMD [ "yarn", "start" ]
+CMD [ "npm", "start" ]