Do not tag intermediate images to allow for automatic pruning

[meyerj]

Docker images need to be cleaned up from time to time to not fill up the file system with obsolete images. This is typically done by running docker image prune or docker system prune. This command does not remove tagged images by default, unless called with -a. The latter is typically not what you want, because it also removes all the images pulled from Docker Hub or other remote registries.

In fact, there is no need for Rocker to assign tags to the intermediate images. The image IDs work just fine.

This patch also renders #52 obsolete.

[tfoote]

The CI on this is failing on Python 3.5 https://travis-ci.org/osrf/rocker/jobs/583145753

$ xvfb-run nosetests --with-coverage --cover-package rocker --exclude test_nvidia_glmark2
E..EE.EEE......EEE.E.EE
======================================================================
ERROR: test_device (test_core.RockerCoreTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/osrf/rocker/test/test_core.py", line 92, in test_device
    self.assertEqual(dig.build(), 0)
  File "/home/travis/build/osrf/rocker/src/rocker/core.py", line 122, in build
    output_callback=lambda output: print("building > %s" % output)
  File "/home/travis/build/osrf/rocker/src/rocker/core.py", line 87, in docker_build
    image_id = match[1]
TypeError: '_sre.SRE_Match' object is not subscriptable
-------------------- >> begin captured stdout << ---------------------
Writing dockerfile to /tmp/tmp3y84_107/Dockerfile

The Match appears to only work with subscriptable elements in 3.6 and higher:
https://stackoverflow.com/questions/52404656/sre-sre-match-object-is-not-subscriptable

[codecov]

Codecov Report

Merging #60 into master will increase coverage by 1.15%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   83.33%   84.49%   +1.15%     
==========================================
  Files           5        5              
  Lines         390      374      -16     
==========================================
- Hits          325      316       -9     
+ Misses         65       58       -7

Impacted Files	Coverage Δ
src/rocker/nvidia_extension.py	89.04% <ø> (+1.71%)	⬆️
src/rocker/cli.py	20.45% <ø> (-0.38%)	⬇️
src/rocker/os_detector.py	94.11% <100%> (-0.48%)	⬇️
src/rocker/core.py	90.27% <100%> (+1.78%)	⬆️
src/rocker/extensions.py	100% <0%> (ø)	⬆️

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[tfoote]

Thanks this is definitely cleaner. I fixed up the regex usage in python 3.5 to pass CI.