Update dockerfile to use slim image base

[mbeacom]

- What I did

Modified the Dockerfile to use http://label-schema.org/rc1/ syntax to label the docker image and switched the base image to use the latest slim 3.6 base image.
This decreases the docker image size from 740MB to 278MB without any usage/compatibility changes.

- How I did it

• Removed deprecated MAINTAINER directive.
• Added new LABEL directives adhering to label schema specific to the repository.
• Modified the base image from python:3.6 to python:3.6-slim
• Modified installed dependencies to install only whats necessary and purge them following build of package.
• Switch from cloning the github repository to using the existing repository, since we're already in the repo.

- How to verify it

• Run make docker-build

- Description for the changelog

• Switch Dockerfile to python:3.6-slim from python:3.6 to decrease size from 740MB to 278MB.

- Cute Animal Picture

[fubuloubu]

LGTM thanks

[DavidKnott]

For make docker-build to work I needed to add -t viper ., @mbeacom does it work without that for you?

@docker build -t viper . \
		--build-arg VCS_REF=`git rev-parse --short HEAD` \

[mbeacom]

@DavidKnott Yes, it seems to work fine on my end:

~/C/viper   slim-docker  make docker-build           Sat Dec 23 14:58:37 2017
Sending build context to Docker daemon  2.009MB
Step 1/8 : FROM python:3.6-slim
3.6-slim: Pulling from library/python
c4bb02b17bb4: Already exists

I can add the tag to it, though! Wasn't sure if we wanted to tag it or not.

[DavidKnott]

@mbeacom Thanks a lot! merging this in.