title: bash does not work using Docker's SHELL instruction
body: <p>I need to use <code>bash</code> in the scripts of a CI/CD pipeline, so I'm trying with this simple image:</p>
  
  <pre><code>FROM alpine:3.4
  RUN apk update -q &amp;&amp; apk add --no-cache bash -q
  SHELL ["/bin/bash", "--login", "-c"]
  RUN echo $0
  </code></pre>
  
  <p>What I get in my terminal is this:</p>
  
  <pre><code>$ docker build .
      Sending build context to Docker daemon  2.048kB
      Step 1/4 : FROM alpine:3.4
       ---&gt; b7c5ffe56db7
      Step 2/4 : RUN apk update -q &amp;&amp; apk add --no-cache bash -q
       ---&gt; Using cache
       ---&gt; 18b729da453c
      Step 3/4 : SHELL ["/bin/bash", "--login", "-c"]
       ---&gt; Using cache
       ---&gt; 03ca0df4a543
      Step 4/4 : RUN echo $0
       ---&gt; Running in a65419dafc4b
      /bin/bash
      Removing intermediate container a65419dafc4b
       ---&gt; 2963f9e0e563
      Successfully built 2963f9e0e563
  </code></pre>
  
  <p>After that, I run that container and get this:</p>
  
  <pre><code>$ docker run -it 2963f9e0e563
  / $ echo $0
  /bin/sh
  / $ 
  </code></pre>
  
  <p>(it runs as root but I changed "#" with "$" for this post)</p>
  
  <p>Why is it using <code>sh</code> instead of <code>bash</code>?</p>