Support compiling locally with the new compile service images

[riknoll]

This adds a new secret flag for compiling targets locally using the compile service docker images. The flag is currently hidden because right now this requires you to have pxt-deployment-config checked out locally; I might in the future duplicate the relevant parts of that repo in this one so that we don't have that restriction.

To test docker images locally:

• Build the docker file locally
• Open the pxtarget.json of a target that uses the relevant image (e.g. pxt-microbit for yotta:main and yotta:main-gcc5)
• If necessary, fix the tag in pxtarget.json to match your locally compiled docker image (e.g. in pxt-microbit, you want to change pext/yotta:gcc5 to pext/yotta:main-gcc5)
• Set the GITHUB_ACCESS_TOKEN environment variable to a github access token. Note that this token does not need any permissions at all! It's only used to prevent yotta from getting throttled when cloning public repos, so no need to give access to private repos or anything else.
• Inside the target repo, run:

  rm -rf built/
  pxt buildtarget --localcompileservice
  

The rm -rf above is just to make sure that the local hexcache is cleared. If the hexcache isn't cleared, the docker build might get skipped.