Do not make hidden directory inside builder shim

[JaewonHur]

Previous docker ignore fix (#68) has race bug when syncing files to the buildkit daemon. Upon file request from buildkitd, the FSSync proxy in builder shim checks _checksum variable to check if build context is synced inside VM, and responds differently: i) if not, it forwards a request to the proxy outside to get the file, ii) if synced, it immediately provides the file.

However, since #68 creates a hidden docker directory inside builder shim during sync, if the request for Dockerfile is received before the sync, it just forwards the request outside, which doesn't know that hidden directory.

This PR assumes the directory is created outside, so that such request can be handled.