What is .build-deps for apk add --virtual command? : <p>What is <code>.build-deps</code> in the following command?  I can't find an explanation in the Alpine docs.  Is this a file that is predefined?  Is see this referenced in many Dockerfiles.</p>

<pre><code>RUN apk add --no-cache --virtual .build-deps \
gcc \
freetype-dev \
musl-dev

RUN pip install --no-cache-dir &lt;packages_that_require_gcc...&gt; \

RUN apk del .build-deps
</code></pre>
