Docker build para na etapa 7/9 e cria imagem sem nome

Dockerfile:

FROM node:latest
MAINTAINER Rodrigo Santos
ENV NODE_EMV=production
ENV PORT=3000
COPY . /var/www
WORKDIR /var/www
RUN npm install
ENTRYPOINT ["npm", "start"]
EXPOSE $PORT

Quando rodo o comando de build:

docker build -f Dockerfile -t rodrigosantos/node .

O terminal fica parado na etapa 7:

Sending build context to Docker daemon  6.144kB
Step 1/9 : FROM node:latest
 ---> 6dc0a5fbad51
Step 2/9 : MAINTAINER Rodrigo Santos
 ---> Using cache
 ---> cccefc53a154
Step 3/9 : ENV NODE_EMV=production
 ---> Using cache
 ---> 3765389eaf93
Step 4/9 : ENV PORT=3000
 ---> Using cache
 ---> 936551f5a99e
Step 5/9 : COPY . /var/www
 ---> Using cache
 ---> 439c2585f536
Step 6/9 : WORKDIR /var/www
 ---> Using cache
 ---> d47697baaba8
Step 7/9 : RUN npm install
 ---> Running in 9d8bea14f9b5

Após parar o terminal e rodar o comando para listar as imagens:

docker images

Retorna a imagem criada mas sem nome:

REPOSITORY                  TAG       IMAGE ID       CREATED          SIZE
<none>                      <none>    2b84b379a1ea   2 hours ago      992MB
docker                      latest    0c655f708b2a   8 days ago       216MB
node                        latest    6dc0a5fbad51   9 days ago       992MB
ubuntu                      latest    ba6acccedd29   5 weeks ago      72.8MB
dockersamples/static-site   latest    f589ccde7957   5 years ago      191MB