Stop <command> from converting CMD to "exec form" (JSON array)

[alan-czajkowski]

The following does not work:

<configuration>
  <images>
    <image>
      ...
      <build>
        ...
        <env>
          <ENV_VAR>some_value</ENV_VAR>
        </env>
        ...
        <command>echo $ENV_VAR</command>
      </build>
    </image>
  </images>
</configuration>

because <command> is converted to a JSON array (exec form), and according the documentation http://docs.docker.com/reference/builder/#cmd

Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, CMD [ "echo", "$HOME" ] will not do variable substitution on $HOME.

the documentation mentions this work-around:

If you want shell processing then either use the shell form or execute a shell directly, for example: CMD [ "sh", "-c", "echo", "$HOME" ].

but this is not correct, the documentation is wrong, in order for environment variable interpolation to work in exec form, in the CMD, it must be structured like this: CMD [ "sh", "-c", "echo $HOME" ]

but this cannot be achieved through the plugin because <command>sh -c echo $ENV_VAR</command> results in CMD [ "sh", "-c", "echo", "$ENV_VAR" ] which is broken whereas we want CMD [ "sh", "-c", "echo $ENV_VAR" ] which is the desired result

The conversion of <command> to a JSON array (exec form) should be disabled.

[rhuss]

With #130 being implemented you should be able to use the following in 0.12.0:

<configuration>
  <images>
    <image>
      ...
      <build>
        ...
        <env>
          <ENV_VAR>some_value</ENV_VAR>
        </env>
        ...
        <cmd>
           <shell>echo $ENV_VAR</shell>
        </cmd>
      </build>
    </image>
  </images>
</configuration>

0.12.0 will be out next week.