Use build args to capture build time data

[Cryptophobia]

From @jchauncey on November 10, 2016 17:53

Acceptance Criteria:

• Use the --build-args flag when running docker build to pass in the following items (and more if needed)
• BUILD_DATE
• VERSION

You will need to do the following in the dockerfile to persist the data into the image:

ARG VERSION
ARG BUILD_DATE
ENV VERSION $VERSION
ENV BUILD_DATE $BUILD_DATE

Copied from original issue: deis/fluentd#71