issue #92: inserting environment file to docker instance

This would cover #92. `ENV` script is injected into docker instance, it will be picked up by `/start` script then.

Unfortunately, I could not do end-to-end test, I just can see that `/app/.app-env` appears on docker instance. I'm waiting till https://github.com/progrium/buildstep/pull/18 is accepted and I assume that https://s3.amazonaws.com/progrium-dokku/progrium_buildstep.tgz should be updated after, it that right?

As all pieces in place, it should work fine.