Clone Ere and build docker image from source if workspace not found

If `ere-dockerized` is compiled, and distributed as binary only, the source of Ere wouldn't be found, hence couldn't be the docker image from source.

In this case, we could try to pull the exact revision of Ere as the workspace to continue. But this should be rare case, I'd image if only binary is distributed, the image of `ere-server` will also be prepared already.
