update image to use python3 (#531)
update image to use python3 (#531)

python2 has reached end of life, so replacing it with python3.

In addition, pip (python3 version) comes along with the python3 install
on alpine, so no need to install that explicitly.
ricardorsierra/dockerfiles,jfrazelle/dockerfiles,ricardorsierra/dockerfiles,jfrazelle/dockerfiles,ricardorsierra/dockerfiles,michael-k/dockerfiles-jfrazelle,jessfraz/dockerfiles,michael-k/dockerfiles-jfrazelle,michael-k/dockerfiles-jfrazelle,jessfraz/dockerfiles,jessfraz/dockerfiles,jessfraz/dockerfiles,ricardorsierra/dockerfiles,michael-k/dockerfiles-jfrazelle,michael-k/dockerfiles-jfrazelle,jessfraz/dockerfiles,ricardorsierra/dockerfiles