Add BUILD_TIMESTAMP env var

[solsson]

As an alternative to #22. Jenkins has https://issues.jenkins-ci.org/browse/JENKINS-26520.

To use this to disable caching of a run step (inspired by http://stackoverflow.com/questions/36996046/how-to-prevent-dockerfile-caching-git-clone):

In a build contract compose yml:

services:
  x:
    build:
      context: ../
      args:
        timestamp: ${BUILD_TIMESTAMP}

and dockerfile

RUN BUILD_TIMESTAMP=${timestamp} \
  && my-stuff-that-should-not-be-cached