title: Docker buildkit cache location/size and ID
body: <p>There's Docker buildkit: <a href="https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/experimental.md" rel="noreferrer">https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/experimental.md</a>
  which has an extra option to <code>RUN</code> (amongst others): the <code>--mount=type=cache</code>.</p>
  <p>What I couldn't figure out from the documentation is:</p>
  <ol>
  <li>where is that cache stored?</li>
  <li>how can one delete/inspect that?</li>
  <li>how does buildkit decide what cache goes where? For example if I have the same Dockerfile in two locations, will the caches be the same? What's the key for the cache?</li>
  <li>the ID option is still ambiguous. If I specify the same ID in different dockerfiles, will they refer to the same cache?</li>
  </ol>