title: Build args does not pass in value to docker file
body: <p>I want to build a docker image. And I run</p>
  
  <pre><code>docker build --build-arg project_file_name=account.jar -t account:1.0 .
  </code></pre>
  
  <p>The docker file looks like this (#1)</p>
  
  <pre><code>FROM anapsix/alpine-java:8u172b11_server-jre
  
  ARG project_file_name
  
  MAINTAINER jim
  COPY src/${project_file_name} /home/${project_file_name}
  CMD java -jar /home/${project_file_name}
  </code></pre>
  
  <p>If hardcode the variable, it will look like this (#2)</p>
  
  <pre><code>FROM anapsix/alpine-java:8u172b11_server-jre
  
  MAINTAINER jim
  enter code here
  COPY src/account.jar /home/account.jar
  CMD java -jar /home/account.jar
  </code></pre>
  
  <p>After I build the image with #1 and #2</p>
  
  <p>Using #1, when I docker run, docker tell me it cannot find the specified jar file</p>
  
  <p>Using #2, when I docker run, docker is able to execute the java jar file correctly.</p>
  
  <p>To me both #1 and #2 are same. Just #1 use build-arg variable way and #2 is hardcoding the value. I believe the way I use build-args is incorrect. Can anyone guide me on this?</p>
  
  <p>Regards</p>