Publish images with autotag

[tonglil]

http://plugins.drone.io/drone-plugins/drone-docker/#autotag

Pushes to master should only update latest.

Tags will create the image's tag, and update the image's existing "higher" tags.

action	git tag	image tags created	image tags updated
0.4.0	0.4.0	0.4 and 0
patch	0.4.1	0.4.1	0.4 and 0
minor (backwards compatible)	0.7.0	0.7.0 and 0.7	0
major (breaking changes)	1.0.0	1.0.0, 1.0, and 1
patch	1.0.1	1.0.1	1.0 and 1