这是我的Dockerfile：

FROM ubuntu:14.04
RUN apt-get update
RUN apt-get upgrade --assume-yes
RUN apt-get install wget vim git --assume-yes
# install RVM
RUN apt-get install build-essential curl --assume-yes
RUN curl -L https://get.rvm.io | bash -s stable
RUN echo 'source /etc/profile.d/rvm.sh' >> ~/.bashrc
RUN /usr/local/rvm/bin/rvm-shell -c "rvm requirements"
# install Ruby
RUN /usr/local/rvm/bin/rvm-shell -c "rvm autolibs enable"
RUN /usr/local/rvm/bin/rvm-shell -c "rvm install 2.1.2"
# install Rails
RUN echo "gem: --no-rdoc --no-ri" >> ~/.gemrc
RUN gem install rails -v 4.1.5
# install nodeJS
RUN sudo apt-get install nodejs --assume-yes
EXPOSE 3000

比我建立：

sudo docker build -t="james/rails" .

我收到了这个错误：

Step 11 : RUN gem install rails -v 4.1.5
 ---> Running in 44efc6b7c254
/bin/sh: 1: gem: not found
2014/09/04 18:33:52 The command [/bin/sh -c gem install rails -v 4.1.5] returned a non-zero code: 127