This placeholder file guarantees git will not get rid of the docker-context
directory.

When building our ci images, we use the docker-context directory as "empty
context", ie, a context that does not change in every commit. In this way, only
when the Dockerfiles are modified, the docker build will actually build another
image.  It is ok to use an empty context for our ci images because they only
contain external tools (and no code from this repository).
