How to unset "ENV" in dockerfile? : <p>For some certain reasons, I have to set "http_proxy" and "https_proxy" <code>ENV</code> in my dockerfile. I would like to now unset them because there are also some building process can't be done through the proxy.</p>

<pre><code># dockerfile

# ... some process

ENV http_proxy=http://...
ENV https_proxy=http://...

# ... some process that needs the proxy to finish

UNSET ENV http_proxy # how to I unset the proxy ENV here?
UNSET ENV https_proxy

# ... some process that can't use the proxy 

</code></pre>
