Skip Docker image build on CI when unchanged

Previously we were able to reuse Docker layers from prebuilt images pulled from docker hub.
This has been reducing total build time by 3 hours out of 7~8 hours total.
After Docker 1.10 or so, it is no longer possible and we tend to easily saturate entire Apache's 30 jobs on Travis-CI.

Standard solution as of now is to use docker save/load.
This typically requires automated file upload on CI to some external storage.
Unfortunately it cannot be done with our current Travis-CI account settings.

To workaround this, we can put Dockerfile itself to Docker image and see if it's modified after the prebuild time and skip fresh builds if unchanged.