Docker build - erro npm

Por favor, poderiam me ajudar, pois ao executar o docker build, o mesmo apresenta o erro descrito na saída abaixo.

xxxxx@docker:~/volume-exemplo$ docker build -f Dockerfile -t teste/node .
Sending build context to Docker daemon  6.144kB
Step 1/8 : FROM node:latest
 ---> 8c67bfd7b95b
Step 2/8 : MAINTAINER Anonimo
 ---> Using cache
 ---> f5436a0dabd7
Step 3/8 : ENV PORT=3000
 ---> Running in 75194e57b396
Removing intermediate container 75194e57b396
 ---> 8eb79a5286cb
Step 4/8 : COPY . /var/www
 ---> 164e25030b76
Step 5/8 : WORKDIR /var/www
 ---> Running in dec4fb74df09
Removing intermediate container dec4fb74df09
 ---> 7cb6d63f970d
Step 6/8 : RUN npm install
 ---> Running in 36bbd3c6ce2c
npm notice created a lockfile as package-lock.json. You should commit this file.
npm WARN volume-exemplo@1.0.0 No description
npm WARN volume-exemplo@1.0.0 No repository field.

added 48 packages from 36 contributors and audited 121 packages in 3.903s
found 0 vulnerabilities

Removing intermediate container 36bbd3c6ce2c
 ---> e3f6e5ad87fe
Step 7/8 : RUN npm start
 ---> Running in 6542188ee6c0

> volume-exemplo@1.0.0 start /var/www
> node .

Server is listening on port 3000

Diretório:

jensen@docker:~/volume-exemplo$ ls
Dockerfile  index.html  index.js  main.css  package.json

Dockerfile:

FROM node:latest
MAINTAINER Anonimo
ENV PORT=3000
COPY . /var/www
WORKDIR /var/www
RUN npm install
RUN npm start
EXPOSE $PORT