Proposal: Add docker alpine image to support devicemapper

[zaquestion]

Changes:

• Add execute permissions added to discover.py presence.py, and dl_lib.py
• New Dockerfile.alpine
  • Fewer layers
  • CAdvisor moved to wget (so it can be cached)

TODOs / feedback request.

• [ ] gcc and python-dev are installed, though agent-base installs them and removes for clean up. The sole purpose of bringing them in here is to install grequests which a few of the *.py scripts require. Potentially install grequests higher up and remove the additions here to support it.
• [ ] /usr/bin/python gets linked into /opt/dataloop/embedded/bin/ as the *.py scripts expect it.
• [ ] pick a version for grequests if its to remain here.
• [ ] re-remove python-dev and gcc?

Having come this far I'm fairly interested in completing the changes. If this PR appeals to you I'd be happy to address any feedback.

Related: https://github.com/dataloop/dataloop-docker/issues/17

[sacreman]

Hi, I'd like to merge this but I can't do it directly into master as that builds :latest on docker hub and I think a lot of companies do an auto update.

Is there any chance you could close and resubmit as a pull request into the unstable branch?